Scientists have uncovered a possible biological explanation for the cognitive changes many women experience during pregnancy, commonly known as "pregnancy brain" or "momnesia." These changes include temporary memory issues, such as difficulty recalling names or words. A recent study published in Science Bulletin suggests that high levels of the hormone estrogen during pregnancy may be responsible for these effects by altering a specific brain pathway. The research team used laboratory mice genetically modified to experience estrogen surges similar to those in pregnant women. These mice showed temporary memory problems, but their moods remained unaffected. The issue was traced to a neural connection between the hypothalamus — a brain region that controls bodily functions — and the hippocampus, which is essential for forming memories. The study found that high estrogen levels stimulated neurons in the hypothalamus, which in turn suppressed activity in the hippocampus, impairing memory function. These effects were most noticeable in late pregnancy and could continue into the postpartum period before gradually disappearing. The cognitive changes can impact both short- and long-term memory, including the ability to remember names, words, and familiar information. Dr. Zheng Sun, a professor at Baylor College of Medicine and a lead researcher, noted that approximately 80% of pregnant women report these changes, though the number is based largely on self-reports rather than objective tests. The study clarifies that these changes do not indicate a general loss of intelligence, but rather a subtle and specific impact on certain memory-related tasks. The researchers identified a protein called "estrogen receptor alpha" that transmits hormonal signals directly into hypothalamic cells. These cells contain many GABAergic neurons, which typically send inhibitory signals. In mice, high estrogen levels overactivated these neurons, reducing hippocampal activity and leading to memory problems. Behavioral tests showed that mice with high estrogen performed worse in tasks involving object and spatial recognition. Disrupting the connection between the hypothalamus and hippocampus prevented memory issues, and activating the pathway caused memory problems even without increased estrogen. To support the findings in mice, the researchers tested 70 pregnant women and non-pregnant individuals on various memory tasks. They found that women in late pregnancy showed memory deficits linked to higher estrogen levels. Dr. Yanlin He, another lead researcher, emphasized that higher estrogen levels were associated with poorer memory performance, even after considering other factors. While the human study supports the hypothesis, it does not confirm the same direct cause-and-effect relationship as the mouse experiments, since the specific brain circuit was not directly manipulated or measured in humans.