Handbook of Nutrition and Pregnancy

By Carol J. Lammi-Keefe and Sarah C. Couch

​This easy to use text provides practitioners and researchers with a global view of current and emerging issues concerned with successful pregnancy outcomes and approaches that have been successful or show promise in ensuring a successful pregnancy.  The fully updated and revised second edition expands its scope with topics not covered in the first edition including pregnancy and military service; sleep disorders during pregnancy; the gut microbiome during pregnancy and the newborn; requirement for vitamin D in pregnancy; the environment—contaminants and pregnancy; preeclampsia and new approaches to treatment; health disparities for whites, blacks, and teen pregnancies;  depression in pregnancy—role of yoga; safe food handling for successful pregnancy outcome;  relationship of epigenetics and diet in pregnancy; caffeine during pregnancy; polycystic ovary syndrome;  US Hispanics and preterm births; celiac disease and pregnancy; cannabis use during pregnancy.

The second edition of Handbook of Nutrition and Pregnancy will be a valuable resource for clinicians and other healthcare professionals who treat and counsel women of child-bearing age and pregnant women.

• Language: English
• Publisher: Humana Press
• Release date: Aug 3, 2018
• ISBN: 9783319909882

Book preview

Part INutrient and Health Needs in Pregnancy

© Springer International Publishing AG, part of Springer Nature 2018

Carol J. Lammi-Keefe, Sarah C. Couch and John P. Kirwan (eds.)Handbook of Nutrition and PregnancyNutrition and Healthhttps://doi.org/10.1007/978-3-319-90988-2_1

Body Composition in Pregnancy

Nicholas T. Broskey¹  , Kara L. Marlatt¹   and Leanne M. Redman¹  

(1)

Reproductive Endocrinology and Women’s Health Lab, Pennington Biomedical Research Center, Baton Rouge, LA, USA

Nicholas T. Broskey

Email: [e-mail]

Kara L. Marlatt

Email: [e-mail]

Leanne M. Redman (Corresponding author)

Email: [e-mail]

Keywords

Body compositionGestational weight gainFat massFat-free massTotal body waterBirth weight

Nicholas T. Broskey and Kara L. Marlatt are the co-first author.

Key Points

Entering pregnancy at a healthy weight, and maintaining a healthy weight gain throughout pregnancy, has become increasingly important.

Excessive gestational weight gain, particularly early in pregnancy, has negative consequences on both maternal and infant health outcomes.

The variance in total gestational weight gain is the result of variable increases in fat mass.

Given body composition during pregnancy is different compared to pre-gravid conditions, careful methodological considerations and adjustments are required.

Magnetic resonance imaging and ultrasound imaging have emerged as promising new techniques to measure maternal body composition.

The distribution of gestational weight gain into different adipose tissue depots is an important avenue of exploration in pregnancy research.

Assessment of Body Composition During Pregnancy

Pregnancy is a unique time when the female body is undergoing physiologic and metabolic changes that support fetal development. These physiologic and metabolic changes are recognized in gestational weight gain (GWG), which includes gains in maternal and fetal body mass, as well as growth of placental tissue and alterations in amniotic fluid. Indeed, increased medical attention is placed on optimizing maternal and fetal outcomes and as such, body composition is recognized as an important modulator of these outcomes in a pregnant woman and her offspring.

Common methods used to assess body composition in a pre-gravid female cannot be directly applied to the pregnant female without adequate adjustments or consideration of the drastic deviation in normal body composition. Additionally, while non-invasive and less cumbersome methods of assessing body composition, particularly fat mass (FM) and fat-free mass (FFM), are necessary in research and clinical settings, certain methods are clearly more superior to others. Herein many of the common methods to assess body composition are described, as well as their consideration for measurement in pregnant women. Finally, changes in body composition throughout pregnancy using these techniques are also discussed.

Methods of Assessment

Anthropometry

Anthropometry via skinfold thickness measurement is a non-invasive method of assessing body composition that is suitable for field research given its use of highly mobile, non-specialized equipment. Specifically, skinfold thickness measurements provide an estimated size of the subcutaneous fat depot directly under the skin. The summation of skinfold thickness measurements at particular areas of the body can be used to obtain an estimation of total subcutaneous body fat. Extensive training and expertise of the technician is necessary to accurately assess skinfold thickness and ensure a high level of reliability both within an individual over time (pregnancy) and also between individuals. Moreover, the appropriate use of necessary equipment such as anthropometry tape (Gullick), stadiometer, weight scale, skinfold calipers, anthropometer, and segmometer is critical to locate the anatomical site for measurement, and thereby obtain reliable and consistent results while minimizing measurement error.

Pregnancy presents a unique challenge for assessing skinfold thickness due to both tissue expansion and stretch. The increase in individual skinfold thickness during pregnancy is often greater in underweight women compared to overweight women, and greater at central sites compared to those located in the periphery. Taggart and colleagues demonstrated the absolute change (millimeters) in individual skinfold thickness at seven anatomical sites (i.e., suprailiac, scapular, costal, biceps, knee-cap, mid-thigh, triceps) occurs between 10 and 30 weeks gestation, yet all sites but thigh, showed little change or decreased thickness from 30 to 38 weeks [1]. Similar observations were reported by Pipe et al. using the summation of four anatomical sites (i.e., triceps, biceps, subscapular, suprailiac); however, slight increases were observed up to 36–38 weeks due to greater gains at the suprailiac site [2]. Additionally, the increase in skinfold thickness during pregnancy is often greater, on average, among primiparae than among multiparae women [1].

Few studies use skinfold thickness during pregnancy to estimate total fat mass. Equations to estimate body composition from skinfold thickness also use weight and body composition (FM, FFM). While some published models claim to explain a high percentage of the variability in predicting FM or percent body fat, these models cannot be extrapolated across pregnancy, and instead serve to estimate fat mass only at designated time periods [3, 4]. One important anthropometric study [2] detailed skinfold thickness measures at six different sites across multiple time points before and during pregnancy and related total body fat; however, others have reported that only mediocre multiple correlation coefficients were observed [5]. Similar results have been demonstrated [6, 7], and collectively conclude that use of equations to estimate total body fat from skinfold thickness may be useful in certain groups; however, they are inappropriate for most clinical and research purposes. With precision being maintained, repeated assessment of subcutaneous fat with skinfold thickness throughout pregnancy can be useful in research and clinical settings.

Bioelectrical Impedance

Bioelectrical Impedance (BIA ) is a commonly used non-invasive method that is based on assumptions and relationships in the electrical properties of biological tissues. Typically, BIA is performed by placing electrodes on the ankle and wrist, allowing for the flow of low-amperage current to travel throughout the body. The conductivity of the electrical current is determined by the amount of water the biological tissue contains. Tissues with high water content (e.g., muscle) are more conductive than tissue with less water content (e.g., bone, fat) and, therefore, the volume of conductive tissue can be calculated from the resistance of the electrical signal throughout body parts. BIA, therefore, allows for an estimation of total body water and subsequently, estimations of FM and FFM. A similar principle to BIA is bioimpedance spectroscopy, which allows for estimation of intracellular and extracellular water and thus, total body water (TBW) by summing the two cellular water compartments.

In a validation study for pregnant women, Lof and Forsum [8] utilized wrist-to-ankle bioimpedance spectroscopy during various stages of pregnancy (14 and 32 weeks, 2 weeks postpartum) and reported similar estimates of TBW measured by BIA versus deuterium dilution (the gold standard) early in pregnancy, but the estimate of TBW by BIA was underestimated later in pregnancy.

The inherent problem with BIA is that this measurement of body composition is based on TBW , which changes during the course of pregnancy. Therefore, in addition to fluid shifts associated with pregnancy that have a wide degree of inter- and intra-individual variation, hydration status can also affect BIA measurement [9]. Standardization of time of day is therefore important, as well as understanding the hydration status of the individual. Changes in TBW also happen concomitantly with changes in overall composition, adding to the problem of precision and accuracy, bringing into question the feasibility of BIA to measure body composition throughout pregnancy. Otherwise, BIA is a rapid, non-invasive, and inexpensive method to estimate body composition that is suitable in field settings. Unfortunately, it is unable to decipher between maternal and fetal contributions.

Densitometry

Whole-body densitometry is a non-invasive method to obtain body density of the maternal–fetal unit as a whole. Unfortunately, densitometry is currently unable to separate both the maternal and fetal contribution. Densitometry can be estimated in water (known as hydrodensitometry or underwater weighing), or it can be estimated in air (known as air-displacement plethysmography). Relying on the assumption that the density of FM is constant (0.900 g/cm³) and that FFM density depends on relative contributions of bone, protein, and water, and is estimated as 1.100 g/cm³ in men and women in the pre-gravid state [16], whole-body densitometry commonly applies the equation of Siri to estimate FM [15]:

$$ \mathrm{FM}\ \left(\mathrm{kg}\right)=\left(\frac{\mathrm{Body}\ \mathrm{Mass}}{100}\right)\times \left[\frac{\frac{100}{\mathrm{TBD}}\hbox{--} \frac{100}{\mathrm{D}\left(\mathrm{FFM}\right)}}{\frac{1}{\mathrm{D}\left(\mathrm{FM}\right)}\hbox{--} \frac{1}{\mathrm{D}\left(\mathrm{FFM}\right)}}\right]\to \left(\frac{\mathrm{Body}\ \mathrm{Mass}}{100}\right)\times \left[\frac{495}{\mathrm{TBD}}-450\right] $$

where TBD = total body density, D(FM) = density fat mass (or 0.90 g/cm³), D(FFM) = 1.10 g/cm³

Moreover, densitometry is thereby based on a 2C model of body composition where body mass is assumed to be a function of FM and FFM (combined). Hence, FFM is derived by subtracting the calculated FM from the total body mass of the individual.

The estimation of FM and FFM during pregnancy with 2C models, though, is more complex because of the well-documented changes in FFM density . In the early stages of pregnancy, small changes in FFM are predominantly due to the expansion of maternal tissue; the growth of these maternal tissues minimally affects the density of FFM. In later stages of pregnancy, however, the density of FFM is reduced due to the increased growth of fetal tissues that have higher water content and subsequently a lower density [17]. The accumulation of water in pregnancy is gradual, non-linear, and highly variable in women, and may even plateau or decline in late pregnancy [18, 19]. While some scholars suggest that FFM density is the same at 10 weeks when compared to the pre-gravid state (or 1.100 g/cm³) [4, 20], others suggest that FFM density decreases during the first trimester to approximately 1.099 g/cm³ [10]. Table 1 documents the estimated FFM density throughout pregnancy. To provide more accurate estimates of FM and FFM by 2C models applied at the different stages of pregnancy, the appropriate density of FFM should be substituted into densitometry equations (i.e., Siri). Without adjusting densitometry equations for pregnancy-specific FFM density , FFM will be underestimated and result in an overestimation of FM.

Table 1

Changes in fat-free mass density throughout pregnancy (g/cm³)

Careful steps to enhance measurement accuracy through protocol standardization can indeed be implemented; however, estimating body composition via whole-body densitometry is unfortunately prone to high degrees of variability in pregnancy. Namely, van Raaij et al. demonstrated how the presence of clinical edema can impact hydration and thus, the density of FFM [10]. The temporal relationship of edema adds further complexity to the quantification of FFM density . And while the presence of clinical edema may not be visibly present, the existence of edema may indeed be physically present. Nonetheless, van Raaij et al. [10] and their application of the changes in body composition contributions of Hytten and Leitch to approximate FFM density has become widely recognized as the most well-established method to estimate FFM density during pregnancy [21]. Additionally, while the estimates for FFM density may appear to be relatively consistent throughout the literature, studies are needed to understand the degree to which these FFM densities might be impacted by maternal age, race, and body size as measured by body mass index (BMI).

Underwater Weighing

Underwater weighing is a technique that can apply standard densitometry equations to derive a 2C measure of body mass. With this technique , individuals are completely submerged in a small tank of warm water. The weight of the individual in the water is measured after the individual performs a complete exhalation, to void the lungs and airway of as much residual air volume as possible. Underwater weighing is based off a buoyancy principle formulated by Archimedes, which states that force exerted on an object immersed in water is equal to the weight of the fluid the object displaces [22]. It follows the basic equation:

$$ \mathrm{body}\ \mathrm{volume}=\left({\mathrm{weight}}_{\mathrm{air}}\hbox{--} {\mathrm{weight}}_{\mathrm{water}}\right)/{\mathrm{density}}_{\mathrm{water}} $$

Although on its own, underwater weighing provides a 2C estimate of body composition, underwater weighing has been incorporated into several 4-compartment (4C) models in pregnant women [11, 12]. Underwater weighing has also been modeled together with anthropometric measurements of body composition (skinfold thickness triceps, subscapular, suprailiac) during late gestation and shown to predict 91% of the variance in FM [3]. Altering the hydration constants for body density and TBW has been shown to have little impact on measurements of maternal body composition [18]. As pointed out in the previous discussion of densitometry, the major drawback of hydrostatic weighing lies in the estimate of FFM density. Some women may also find it difficult to maximally exhale and undertake complete submersion with occluded nostrils. Nonetheless, underwater weighing is non-invasive and can be performed longitudinally without undue risk.

Air-Displacement Plethysmography (ADP)

ADP is a safe and relatively fast method of quantifying total body density from estimated total body volume, and is therefore becoming a more widely adopted body composition assessment method in vulnerable populations (e.g., pregnant women, infants). ADP is especially applicable where dual-energy X-ray absorptiometry techniques are harmful or not recommended. The patented BodPod technology (COSMED, Concord, CA, USA) utilizes a dual-chamber model that is based on Boyle’s law, where small contrasts in volume and pressure in each chamber are measured [23]. With correction for thoracic gas and lung volumes by either measurement or estimation, total body volume is determined. Thoracic gas volume is measured via an estimation of both functional residual capacity and tidal volume. While seated inside the measurement chamber, the individual is required to place a plastic tube in the mouth, and with remote coaching from the technician, lightly blow air into a breathing tube connected to the system. Although less accurate, the BodPod software can also predict total lung volume if a direct measurement cannot be adequately obtained by the instrument [24]. An accurate measurement of body volume requires that individuals wear tight-fitting clothing (e.g., lycra swimsuit) to eliminate any residual air from body surface. Table 2 summarizes the relevant BodPod equations that are utilized to estimate FM and FFM.

Table 2

Calculations for total body density using air displacement plethysmography

Advantages of the ADP technique include the high level of safety, therefore allowing repeated measurements across pregnancy, as well as the relative ease and speed of estimating body composition. Disadvantages of the technique in pregnancy are indeed more profound. First, given well-described changes in TBW throughout pregnancy, there is a need to adjust coefficients applied to the estimated FFM density throughout pregnancy to accurately reflect the degree of increased FFM hydration; the manufacturer does not provide these adjustments in the current software. However, as pointed out earlier, the available FFM density adjustments are outdated and were derived from mostly women of normal weight and without evidence of clinical edema. Second, the ADP technique is not portable, and thus not suitable for field research, and is expensive to operate and maintain. A third disadvantage is the inability to assess body density of a pregnant woman independent of the growing fetus and supporting tissues, which limits the assumptions from BodPod to changes in maternal/fetal tissues. While body composition estimation via a 2C model is accepted, the derived estimate of FM and FFM is subpar compared to 3C or 4C models. It is also likely that measures of FM and FFM by ADP are affected by variability in the estimation of thoracic gas volume. While seldom discussed in the literature, obtaining an actual measurement of thoracic gas volume by the BodPod is difficult for some individuals, and thus requires the use of predicted equations. Discrepancies between measured and predicted thoracic gas volume exist and cannot be ignored. For longitudinal measurements, it is important to apply either the measured or estimated thoracic gas volume throughout the assessments to limit this as a potential source of error in the FM and FFM estimations. Finally, residual air may also cause an overestimation of body volume, and adequate attention should be provided to minimize this artifact , especially in pregnant women when the size of the bust and abdomen is changing considerably and could contribute to increased residual air volume.

Imaging

Magnetic Resonance Imaging (MRI)

MRI is an in vivo imaging technique that uses a powerful magnetic field to measure adipose tissue, skeletal muscle, and organ mass. MRI acquisition protocols require that individuals must lie still in a small, enclosed space, often for lengthy periods, while the magnet is running. MRI has several advantages in that it allows for whole-body as well as regional estimates of body composition, and with no exposure to radiation in comparison to DXA. Sohlström and Forsum were one of the first groups to conduct a well-controlled study using MRI in a cohort of 15 Swedish women throughout pregnancy [25]. The MRI showed that of the 7.4 kg gained in pregnancy (to 7 days postpartum), the majority was gained in whole-body subcutaneous adipose tissue. Modi et al. also reported a positive correlation between maternal BMI and total adipose tissue gain [26]. A recent cross-sectional analysis of normal weight and overweight/obese pregnant women using MRI to assess body composition at the third trimester [27] found that overweight/obese women had almost two times the amount of total body FM as well as subcutaneous abdominal fat compared to normal-weight women, with no differences in visceral fat between the two groups.

A recent methodological study was performed in obese women during the early second trimester of pregnancy (15–18 weeks) in order to develop a consistent method for calculating subcutaneous and visceral fat area ratios. By varying the thickness and distance of abdominal slices, the group concluded to produce reliable measurements of subcutaneous and visceral fat using MRI between women, the region of interest and acquisition of abdominal slices should be centered above the uterine fundus [28].

MRI research is highly promising in regard to body composition analyses in pregnant women, and will be critical to advance the understanding of the changes in regional fat distribution, particularly fat accumulation within the abdominal compartment. However, MRI is not without limitations. The most obvious limitation is discomfort attributed to being confined in a small space and position for the scanning protocols. Abdominal protocols, while short, can be very difficult and uncomfortable for pregnant women, especially in the later stages of pregnancy; some protocols require a 15-s breath hold to minimize movement of the chest cavity. In addition, limitations to the field of view will limit accurate abdominal scanning in some women. The MRI scanners can also be noisy; however, newer 3.0 tesla magnets are equipped with sound dampeners that reduce noise, and individuals are provided ear protection and plugs as well. The obvious limitation is cost. Many clinical centers charge approximately $600 USD per 30-min of scanning time (sufficient for an abdominal scan), then the cost for analysis of the images is an additional expense. Manufacturer differences between scanners can introduce problems in standardizing hardware and software among clinical centers. To date, there are no current publications reporting the use of MRI to assess changes in body composition throughout pregnancy; however, two studies employing a whole-body scanning protocol are currently ongoing (MomEE: NCT#01954342; LIFT: NCT#01616147). Finally, it should be noted that safety of using MRI during the first trimester is questionable and, therefore, estimates of changes in body composition across pregnancy with MRI is only possible from the second trimester onward.

Dual-Energy X-Ray Absorptiometry (DXA)

DXA scans are used to measure total body composition by emitting X-ray beams overhead and throughout the entire body while lying supine. While designed to measure bone mineral density, DXA can also provide measurements of total and region-specific FM and FFM. Since DXA scans emit radiation considered harmful to a developing fetus, DXA scans can only be conducted before and/or after pregnancy. A pre-gravid assessment of body composition with DXA, which provides measurements of bone mineral content as well as FM and FFM, can be used in conjunction with other measures performed during pregnancy to compute body composition from equations for either 3C or 4C body composition models. For example, an elegant study by Butte and colleagues used 4C modeling to compute change in maternal body composition throughout pregnancy and postpartum [29]. With the aid of DXA prior to pregnancy to estimate bone mineral content, Butte showed that accretion of TBW, FM, and FFM related linearly to gestational weight gain and that gestational weight gain above Institute of Medicine (IOM) guidelines led to excess postpartum fat retention. Similarly, Kopp-Hoolihan et al. used a 4C model with a DXA estimation of bone mineral content and observed 4.1 kg of body fat was deposited by 36 weeks of gestation [14]. Estimates of the changes in FM and FFM by DXA are primarily hindered by radiation exposure, and secondarily by the rapid changes occurring in all these tissues and changes in body water. Therefore, estimates of body composition in pregnancy by DXA should be interpreted with caution.

Total Body Water (i.e., Hydrometry )

Total body water (TBW) of the combined maternal–fetal unit can be measured by stable isotope deuterium (²H) dilution methods . The basic principle of the labeled water technique is that after an ingested deuterium oxide (²H2O, or D2O) quantity equilibrates within the TBW pool above the level naturally present (i.e., enrichment), it is eliminated from the body as urine and/or saliva. As such, saliva and/or urine measurements are taken just before ²H2O consumption and repeated up to several hours post-dose delivery (normal equilibration takes 3 to 5 h). An increased level of ²H2O appears in saliva and urine. Pre-dose samples are compared to post-dose samples to calculate TBW, FFM, and ultimately FM.

In a normal weight, pre-gravid female, FM can be derived by (body weight–(TBW/HC)), where HC is the hydration constant of FFM (or TBW:FFM ratio, i.e., 72.3–72.4%) [30]. The true HC proportion, however, can range anywhere from 67% to 80% [15, 31], with the HC proportion likely increasing with adiposity [32, 33]. As was previously described of whole-body densitometry adjustments by van Raaij and others (Table 1) with regard to decreases in total body density resulting from variable increases in FFM hydration throughout pregnancy [32], similar HCs have been quantified throughout pregnancy in different populations, as shown in Table 3. As a precaution, these adjustments should not be utilized as definitive HCs for all populations, as HCs likely vary depending on ethnicity and racial backgrounds, as well as in obese pregnancies. Overconfidence in such adjustments may lead to errors in the estimation of body composition. Adjusted HCs developed by both the van Raaij and Hopkinson groups may improve estimations of FM and FFM during pregnancy [10, 11].

Table 3

Hydration constants throughout pregnancy reported in the literature

Advantages of TBW measurements to estimate FFM in pregnancy is the use of the stable isotope. The isotopes are not harmful to the human body and highly portable, making this method suitable for field research. Disadvantages, like other 2C models in pregnancy, include the relative assumptions that need to be applied or corrections for these assumptions for pregnant women using relevant published literature . Unfortunately, derived hydration values are likely population specific and cannot be easily applied beyond the estimated population.

Ultrasound

Ultrasound is a simple technique available in most hospitals that involves the production of sound waves at varying frequencies to measure body composition during pregnancy. Ultrasound applied to measure subcutaneous fat thickness is highly correlated with measurements of caliper-based skinfold thickness, but it is considered to have a slight advantage over caliper-assessments in that it avoids the issue of skin extension that is problematic when using skinfold calipers in obese or pregnant populations [34, 35]. Ultrasound has been used in longitudinal studies to assess thickness of visceral as well as subcutaneous fat in both pregnant [36] and non-pregnant women [37]. The latter study demonstrated a correlation between intra-abdominal thickness from ultrasound and visceral adipose tissue estimated from computed tomography, whereas the former concluded that visceral adipose tissue increased over time during pregnancy. Visceral fat thickness measured with ultrasound in early pregnancy (11–14 weeks) was shown to correlate with several metabolic risk factors [38]. Although it is a safe, simple, and non-invasive technique, ultrasound is lacking because there is no standard protocol for pregnancy, and it has yet to be validated with any MRI measurements. Another limitation with ultrasound is the ability to derive only site-specific measurements of adipose tissue thickness and not measurements of whole-body composition. More research is needed to assess ultrasound validity throughout the course of pregnancy to more precisely measure body composition changes.

Comparison of Different Compartment Models (2C, 3C, and 4C)

Maternal body composition can be estimated by taking into account the tissue partitioned into 2-, 3-, or 4-body compartments. Referred to as the 2C, 3C, or 4C models, each model offers advantages over the other. In the 2C model , the body mass is simply partitioned into FM and FFM, where FFM encompasses TBW, bone, protein, and non-bone mineral mass and the remaining mass is estimated as FM. Changes in FFM hydration throughout pregnancy require FFM density adjustments when using 2C models like skinfold thickness, ADP, underwater weighing, and BIA. While equations predicting body composition from skinfold thickness are not ideal, skinfold thickness is easy and fast to administer and cost-effective if large sample size assessment is required. While more costly assessments like ADP, underwater weighing, and BIA can offer advantages over skinfold thickness, assumptions of FFM hydration make the 2C model in pregnancy flawed. Additionally, such assumptions are based on decades-old studies that are likely not applicable to all populations. However, if a 2C model is the only available model, adjustments using van Raaij and colleagues are recommended [10].

While a 4C model is the ideal model of body composition estimation, the 4C model is not possible in pregnant women due to the inability to derive bone mineral data from DXA during gestation. Several studies have applied the 4C model successfully by measuring bone in the pre-gravid and postpartum states [11, 12]. However, it should be noted that since more than 50% of pregnancies in the US are unplanned, some of these assessments have occurred more than 1 year prior to the index pregnancy [13], therefore limiting the use of the method in most cases. Research has demonstrated, however, that 3C models , which include the addition of TBW estimation, may indeed be adequate to estimate body composition during pregnancy [11, 14, 15]. Specifically, equations derived by Pipe et al. [2] and Siri [15] are likely the optimal method for deriving an improved 3C model.

Gestational Weight Gain

What Is Gestational Weight Gain?

Normal physiologic and metabolic changes occur during pregnancy and are related to variable growth rates of maternal, placental, and fetal components that contribute to increased gestational weight gain (GWG), or weight gain from conception to delivery. Usually, pre-gravid weight is self-reported. As pregnancy progresses, the accumulation of FM, FFM (i.e., protein accretion), TBW, and minerals is deposited in the fetus, placenta, and amniotic fluid, which contribute to the fetal component of GWG or ~35% of total GWG [39]. Conversely, the developing uterus and breast tissue, as well as extracellular fluid, blood, and adipose tissues, contribute to the maternal component, or ~65% of total GWG [39].

The recent Institute of Medicine Report on the 2009 Guidelines for Recommended Weight Gain during Pregnancy defined recommendations for total weight gain in pregnant women according to the different classes of pre-gravid BMI, as shown in Table 4. While the first trimester is usually characterized by only a slight total weight gain of approximately 1–4 lbs. (or 0.5–2.0 kg), the second and third trimesters feature the predominance of the weight gain during gestation. Figure 1 showcases early and late GWG contributions to maternal and fetal units. Gestational weight gain is inversely related to pre-gravid BMI. For example, women with normal to underweight pre-gravid BMI are recommended to gain weight at a rate of approximately 1 lb. (~0.4–0.5 kg) per week during the second and third trimesters, with overweight women recommended to gain 0.6 lbs. (~0.3 kg) per week, and obese women 0.5 lbs. (~0.2 kg) per week [40]. The current guidelines state that women with GWG that exceeds these recommended ranges are more likely to retain weight postpartum and are at increased risk for subsequent obesity. Indeed, abnormal or excessive GWG is a strong predictor of pregnancy outcomes for both women and infants. For example, the odds of developing abnormal glucose tolerance in the third trimester of pregnancy are greater with higher rates of GWG [41]. Maternal obesity and weight retention postpartum are also associated with excessive GWG [42]. In addition to these maternal outcomes, excessive GWG is associated with infants born large for gestational age, and excessive neonatal and infant weight [43–45].

Table 4

Recommended total weight gain ranges by pre-gravid BMI during pregnancy

aCalculations assume a 1- to 4- lb. (or 0.5–2.0 kg) weight gain during first trimester

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Fig. 1

Components of gestational weight gain (GWG) since last menstrual period. Maternal and fetal components of GWG are noted. Panel (a) represents Early GWG (0–<24 weeks) and Panel (b) represents the Total GWG (0–40 weeks). Late GWG contributions are simply the difference between Total and Early GWG. The stated contributions to GWG are from those of a healthy BMI woman who gains within the recommended IOM weight gain guidelines throughout pregnancy. Women who gain above the recommended guidelines are expected to contribute larger GWG in both the maternal and fetal units. (Percentages adapted from Pitkin RM. Nutritional support in obstetrics and gynecology. Clin Obstet Gynecol. 1976 Sep;19(3):489–513)

What Causes GWG?

GWG can be brought about by a variety of factors . The physiological alterations discussed later in this chapter are some of the contributing factors, but others include age, race, and pre-gravid BMI. Increased maternal age has been found to be associated with an increased risk for low birth weight and small-for-gestational-age children [46, 47]. Furthermore, Gross and colleagues found that obese women ≥35 years old had inadequate GWG compared to younger women (25–29 years) [48]. How these factors and differences among obesity classes affect birth outcomes has yet to be elucidated. Race is another factor that influences GWG; a large epidemiology study of approximately 53,000 women found that black women were significantly more likely than white women to gain less than 15 lbs., but less likely than white women to gain greater than 34 pounds [49]. Further, the IOM noted a study of 913,320 women between the years 1995 and 2003. Asian and non-Hispanic black women were more likely to gain 0–9 kg while Hispanic and non-Hispanic white women were more apt to gain more than 20 kg during pregnancy [40]. Larger epidemiological studies that control for health disparities are warranted, specifically comparing differences in GWG across races.

Recent increases in maternal pre-pregnancy BMI has brought to light a number of studies showing that maternal pregnancy outcomes are worsened with increasing degrees of maternal BMI at conception [50, 51]. Generally, GWG is inversely proportional to maternal BMI status [49] The authors found that obese women tended to gain less weight than normal weight or overweight women; however, a quarter of the obese women still gained greater than 35 pounds. As detailed earlier, women who gain weight above the normal recommended IOM Guideline’s weight gain ranges during pregnancy are at increased risk of experiencing adverse maternal outcomes prenatally, at delivery, and postpartum.

Implications of GWG on Maternal Physiology and Outcomes

During Pregnancy

Among the well-studied adverse prenatal maternal outcomes that result from excessive GWG are gestational diabetes mellitus (GDM) and impaired glucose tolerance, as well as pregnancy-associated hypertension (including preeclampsia and eclampsia). Although pregnancy is frequently accompanied by a pronounced physiological decrease in peripheral insulin sensitivity, the combination of decreased peripheral insulin sensitivity and beta-cell dysfunction can lead to the development of abnormal glucose tolerance during pregnancy, or GDM. Indeed, women who enter pregnancy as obese, as well as women whose GWG is above the ranges recommended by the IOM Guidelines, tend to develop more pronounced insulin resistance and abnormal glucose tolerance and are at greater risk for GDM than are non-obese women [52–56]. Some studies even report that women whose GWG was below the recommended range had a higher likelihood of GDM [56–58]. However, there have been studies finding no significant association between GWG and glucose tolerance [59–61].

Weight gain above recommended ranges might also increase risk of experiencing pregnancy-associated hypertension. While the association between GWG and hypertensive conditions remains unclear due to limited and inconclusive data [54, 57, 58, 62, 63], some studies (all rated fair or poor in quality) have reported such an association does exist [58, 63], with others lack consistent control for confounding.

Interestingly, the timing of GWG may influence different physiological and metabolic factors throughout pregnancy. Specifically, GWG above the recommended guidelines in the first trimester has been shown to be predictive of excessive GWG for the entire pregnancy [64]. Women with a normal-weight preconception BMI have a 70% probability of excess total GWG when excess weight gain is experienced in the first trimester, while overweight and obese women have a 90% probability of excess total GWG [65]. While research is limited, excess GWG, independent of total GWG, has been shown to be associated with impaired maternal glucose tolerance later in pregnancy [66] and greater infant adiposity at birth [65], therefore making future adherence to the recommended weight gain guidelines possibly more critical in these early stages of pregnancy.

At Delivery

Excess GWG has also been suggested to increase the risk of complications during labor and delivery, as well as increased likelihood of cesarean section. The evidence for an association between GWG and cesarean delivery is inconsistent, however, in part because of failure in some studies to adjust for route of prior delivery among multiparous women [67]. Moreover, moderate evidence exists on the association between excess GWG and cesarean section over the last decade [63, 68–73], while several recent studies failed to find an association [57, 60, 74]. Additionally, pre-gravid obesity by BMI categorization has been shown to place women at higher risk of cesarean delivery [60, 70–72, 74].

Postpartum

In the postpartum period , potential consequences of excess GWG during pregnancy include weight retention, decreased lactation performance, and postpartum depression, likely as a consequence of weight retention.

Higher GWG is associated with greater postpartum weight retention, yet many studies fail to consistently adjust for dietary intake, physical activity, and breastfeeding behavior [67]. Nonetheless, subsequent postpartum weight retention increases the risk of moving into a higher BMI category regardless of subsequent pregnancies, thereby increasing the risks to the woman and her fetus during subsequent pregnancy, and to the woman’s own longer-term health and risk of cardiovascular disease, type 2 diabetes, cancer, and mental health.

While data is inconclusive, unsuccessful lactation, or decreased lactation performance, may also result from excess GWG during pregnancy. One study has examined the relationship between GWG and lactation and did not find any relationship between GWG and either milk quality or quantity [75]. While obese women have been reported to have shorter breastfeeding duration regardless of GWG [76–79], the evidence for any association between GWG and duration of exclusive or any breastfeeding was rated weak; evidence that low weight gain is associated with decreased initiation of breastfeeding was rated moderate.

Implications of GWG on Infant Physiology and Outcomes

Excess GWG

Historical examinations of weight gain in pregnancy conclude that very little weight gain occurs in the first trimester and weight thereafter increases linearly until delivery [40]. Following the publication of the IOM recommendations for weight gain in pregnancy, the impact of total GWG on maternal and infant outcomes has received increasing attention. Epidemiological studies show that excessive GWG significantly increases the risk for large-for-gestational-age (LGA) infants [80, 81]. A recent study of 650 pregnant women reported that in comparison to adequate GWG throughout pregnancy, excess GWG through 20 weeks, regardless of the change in weight later in pregnancy, significantly increased the risk of LGA infants [82]. Furthermore, a physical activity intervention for healthy management of GWG found that neonates born to women with excessive GWG early in pregnancy not only exhibited greater birth weights, but had significantly more body fat at birth compared to those neonates born to women with adequate GWG or excessive GWG later in pregnancy [83]. A recent study from Overcash et al. [84] suggests that early GWG in obese women of as little as 2 pounds in the first trimester (12–14 weeks) is a strong predictor of exceeding GWG values throughout pregnancy, and that second and third trimesters have the highest relative effect on total GWG. Knabl et al. also reported similar findings in even earlier periods of 8–12 weeks [65]. These studies suggest the timing of GWG may be important for infant outcomes and, in particular, that early gestational weight gain may be more influential for infant