I haven't yet got into the Nessa Carey book, but I've found some notes I made while watching a set of lectures on DVD by Robert Sapolsky. These lectures are aimed at intelligent/interested laypeople rather than specialists, so are somewhat (and perhaps necessarily) simplified. Moreover, they're my notes which are probably even more simplified. (They're also long - I've never figured out how to write succinctly.) Obviously, any errors are mine:
Maternal stress ? increase in the level of stress hormones, glucocorticoids, which readily cross the placental barrier. This can affect the growth of the foetus, including the brain, because the foetus' system reacts to the flight or fight hormones in much the same way as the mother's ?activities not essential to survival are slowed down. Chronic maternal stress can result in a baby with a smaller than expected brain.
What are the consequences of being born with a smaller brain as a consequence of chronic exposure to high levels of glucocorticoids? One part of the brain that is involved with turning off glucocorticoid secretion at the end of a stress episode is smaller than average. So the baby doesn't have a system that's efficient at controlling glucocorticoid levels. If that baby is a female and subsequently becomes pregnant, her foetus is more likely to experience similar high levels of glucocorticoids, and so on. This is a trait that isn't genetic in a Mendelian sense. It's a long-lasting effect that can transcend many generations, before gradually diminishing.
Maternal nutrition ? there are mechanisms that preferentially feed the foetus in cases where maternal nutrition is low. However, extremely low maternal nutrition can have long-lasting effects on further generations. In the winter of 1944-45, the Nazis were occupying Holland but food resources in the country were already low. The Nazis took all the food for themselves and that, combined with a harsh winter that prevented Allied food supplies getting through, resulted in about 10,000 deaths with the primary cause of malnutrition. After Holland was liberated, food supplies returned to normal. Foetuses during this famine, especially in the second and third trimesters, were affected because during that phase of development the metabolism system is developing. They were born smaller than expected, because of poor maternal nutrition, but they also went on as adults to have smaller children. The cause of this is the development of a very thrifty metabolism, unusually efficient at storing away nutrients and salt. The people who were foetuses at the time of the famine were later more at risk for adult-onset diabetes, obesity, and hypertension. When they became pregnant, their metabolic system reduced the level of nutrition reaching their foetus.
If anyone has managed to read through all of that and wants to reach for their red pen, then please do so. One of the problems that I, and probably many other people, have is that our learning can often be just one-way, with no feedback as to whether we're on the right track.