Pregnancy requires many adaptations to new situations. These changes are accompanied by massive neuroendocrine changes leading to adaptations organized by the mother’s brain. These changes are necessary for the different phases of pregnancy and motherhood.
First optimization is needed for the growth and protection of the fetus, next parturition has to be timely and uncomplicated, next maternal behavior such as caring and defending the young is important together with the start of lactation. It’s not easy to understand the complex interaction between female- and pregnancy hormones and their interaction with the maternal brain.
At the start of pregnancy the brain controls the increased appetite necessary for the pregnancy and the expansion of the blood volume as well as deeper breathing. The brain also prevents further ovulations through the pituitary gland.
The most important female hormones secreted in massive amounts during pregnancy are oestrogen en progesterone. They are very lipid soluble and easily penetrate the brain. Their ratio change the balance between inhibition and excitation. The stages of the pregnancy are signaled to the brain by the pattern of their secretion. Labor starts after the progesterone to oestrogen ratio strongly declines.
The fetus is during pregnancy protected from stress hormones such as cortisol by “in-built” mechanisms. The stress system (HPA-axis) activity is reduced during pregnancy, the responsiveness of the system to a wide range of physical and psychological stressors is reduced. If all this fails, the placenta produces an enzyme that acts as a barrier against maternal stress hormones. It converts these stress hormones (cortisol) to inactive metabolites.
The other most important female hormone for pregnancy and it’s effects on the brain is Oxytocine. When the ratio of progesterone to oestrogen drops, oxytocine is released and stimulates expulsive uterine contractions (labor), it reduces the anxiety of the first exposure to the screaming and noisy newborn, it encourages maternal behavior and the oxytocine pulses due to nipple sucking starts the breast feeding. Prolactin also increased at the end of pregnancy which is also important for breast feeding and maternal behavior.
This is just a short introduction to all the different complex systems and adaptations needed with pregnancy. If your interested the most extensive review was published in Neuroscience in January 2008. For a short and comprehensive explanation you can freely read online: 12. The maternal brain by Professor John A Russell, Laboratory of Neuroendocrinology, Department of Biomedical Sciences, University of Edinburgh.
Brunton, P., & Russell, J. (2008). The expectant brain: adapting for motherhood Nature Reviews Neuroscience, 9 (1), 11-25 DOI: 10.1038/nrn2280