The continuous ebb and flow of steroid hormones prompts brain cells to grow many tiny protrusions. Somewhat similar in appearance to thorns on the stem of a rose, these nubs are called dendritic spines. They add surface area to an existing neuron, allowing for more synaptic contact and therefore better information processing. Such spines can grow on a neuron after hormonal stimulation as well as after repeated bouts of stimulation from nearby connecting neurons.
Our lab has built on previous findings from the Rockefeller University showing that dendritic spine densities in the hippocampus increased in concert with the hormonal changes of a female rat's estrus cycle, which is similar to the human menstrual cycle. Best known for its role in memory, the hippocampus also supports maternal behavior. Even after just a few hours of elevated estrogen, the growth was dramatic.
But we learned that the spines are not caused simply by the presence of estrogen. We tested three groups—late-pregnancy females, females treated with a drug that mimics late-pregnancy hormones and females that had recently begun lactating—and saw that all three showed significant increases in dendritic spine concentrations. Unlike the other two groups, lactating females have very low levels of estrogen. We believe that although a mother's hormones initiate spine growth, the process is maintained by the many stimuli a child generates.
With such a thorough remodeling in progress, it is no wonder that many women complain of “pregnancy brain.” The collateral damage of these changes might include an occasionally faulty memory. Human moms experience postpartum memory deficits, too, as work by clinical psychologist J. Galen Buckwalter of the University of Southern California and his colleagues suggests. They found that on cognitive tests of memory for words and numbers, pregnant women and new mothers fared worse than nonpregnant women of about the same age. Their performance on tasks unrelated to child care seemed to suffer.
For the most part, though, the finished product will more than make up for the hiccups a mother may experience as her brain restructures itself. Producing an offspring requires a mother to jeopardize her own health, safety and survival, so her behavioral system kicks in to protect and defend that investment. With the landscape of her brain buffeted by the hormones of pregnancy and pressures of motherhood, she emerges more efficient and geared for survival.
For Liz, the compensation for the downsides of motherhood comes not just from science but also from the heart. By the time we finished writing this article, she had given birth to a healthy baby girl. All the neurobiology in the world pales in comparison to that blissful, ineffable bond that exists between a mother and her baby. Science may explain the maternal brain, but the real marvel—especially when you are gently tucking the blanket around your baby's chin as she sleeps in your arms—might simply be the beauty of a new child's existence.