What is pregnenolone? It is a steroid hormone but is not a reproductive hormone like estrogen or testosterone.  Pregnenolone is also made from cholesterol, which makes it a steroid hormone. It is also considered to be a pro-hormone,’ because it can convert into other hormones depending on the needs of the body. Recently, it has been discovered that pregnenolone is also produced in the brain and spinal cord, which makes it very potent on stimulating the central nervous system and having an impact on the brain.
Pregnenolone is helpful for memory. It is stimulating to the brain for memory and learning, but at the same time, it is also neuroprotective.  The brain is very complicated, and pages could be written on the process of memory foundation and degradation. What we are trying to say here is, pregnenolone helps with learning and also preserving memory by protecting brain cells.
Other helpful uses for pregnenolone:


Memory: short-term
Brain fog
Mental energy and mental motivation
Helpful for learning new information
Mood
Focus

Pregnenolone, like most hormones declines with age. However, pregnenolone can also drop in response to high levels of chronic stress.  Whether that stress is mentally induced such as dealing with a family member's illness, PTSD or a stressful high paced life. Or physical stress such as intense daily cardiovascular exercise can diminish levels of pregnenolone.  
This is also known as ‘pregnenolone-steal.’  In times of chronic stress, the body will shunt the production away from the other steroid hormones to produce more cortisol.  For women, the body will shunt production away from progesterone to make more cortisol.  
You can test for pregnenolone levels in the blood, but the lab reference ranges are so vast that most everyone will fall in normal ranges.  Quest has a reference range of 22-237 ng/dL, and LabCorp’s is anything less than 151 ng/dL.
Both are broad reference ranges. I like to see pregnenolone 80-100.  If a pregnenolone blood test is less than 80 ng/ dL, I will treat the patient with pregnenolone.  And after being on the pregnenolone for 1-3 months, I will retest the blood work to see where their levels are and how they are feeling.  
As we all know, too much of a good thing' is not-so-good.  Taking too much pregnenolone can have side effects.  There is not a lot of research on pregnenolone.  There are some animal studies but not any real prolific human studies. For the layperson, there is not a lot of information about pregnenolone online. There are some contradictory statements on dosing, side effects, and safety.

What I can tell you from my experience with patients, there are relatively little side effects with pregnenolone, as long as you keep the dosage uniquely prescribed for the patient based on their symptoms and the blood work readings.  
Pregnenolone is available over the counter/online without a prescription.  Many pregnenolone doses are just way too high.  I have had new patients walk into my office, and they are on 100mg or more of pregnenolone per day.  There are a few exceptions based on