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Study finds stress in pregnancy reduces chances of having a boy
The researchers also found that the amount of social support a mother received from friends and family made a difference too.
Forgetfulness and mental fog during pregnancy – sometimes referred to as "momnesia" or "pregnancy brain" – may have a biological explanation, according to new research.
The findings were published last month in the journal "Science Bulletin."
‘Mommy brain’ may be explained
Big picture view:
Researchers in Texas and China used mice to mimic the elevated estrogen levels seen during pregnancy. The animals developed temporary memory problems without changes in mood.
The scientists found that sustained high levels of estrogen during pregnancy can disrupt a specific brain pathway involved in memory.
"The findings offer a biological explanation of how exposure to high-level circulating estrogen can temporarily impair memory."
Tests of 70 women found memory difficulties were most apparent during late pregnancy. (Credit: Getty Images)
The researchers found that high estrogen increased activity in GABA-releasing neurons in the lateral hypothalamus. Those neurons project to the hippocampus, where they can affect memory-related activity.
When researchers blocked the pathway between the two regions, the mice no longer experienced the estrogen-related memory problems. Activating the pathway, meanwhile, caused memory problems even without elevated estrogen.
Memory difficulties most apparent during late pregnancy
Dig deeper:
The team also assessed memory performance in seventy women, including non-pregnant controls and pregnant women in each trimester. They also used a battery of cognitive tests to assess long-term memory, short-term memory, and working memory.
What they're saying:
"Overall, the pregnant group performed worse on simple tests that rely heavily on memory, which is correlated with circulating estrogen levels, while being less affected on more difficult tasks that involve more complex executive functions. These results support that ‘momnesia’ is real," Dr. Zheng Sun, an associate professor of medicine at Baylor College of Medicine specializing in endocrinology and molecular biology and one of the leaders of the study, told FOX Local.
Memory difficulties were most apparent during late pregnancy and were associated with higher estrogen levels, the study found.
Findings suggest memory changes are temporary
The other side:
Still, the findings do not suggest that pregnancy causes a broad decline in intelligence or cognitive ability. The researchers described the changes as temporary but did not establish how quickly memory returns after childbirth.
They also emphasize that the study did not directly manipulate or measure the specific neural circuit in pregnant women, suggesting that the human findings support the hypothesis but do not establish the same level of causality as the mouse experiments.
What they're saying:
"I don’t want this to be overinterpreted as suggesting that pregnancy is bad for the brain," Sun added. "The current tests suggest that the effects in humans are modest, and the mouse data suggest that they are relatively short-lived. I hope this work will encourage greater attention to women’s health, particularly conditions and physiological changes that are unique to women."
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Previous studies link cognitive changes during pregnancy
The backstory:
Previous studies have shown that pregnant women may report experiencing some degree of forgetfulness or cognitive changes.
One study, published last year in "BMC Pregnancy and Childbirth" and archived by the National Library of Medicine, suggested that pregnancy modestly affected verbal memory and attention compared to non-pregnant controls, with indications that the decline persisted postnatally.
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Another study, published last year in Nature Communications, found that gray-matter volume decreased by up to 4.9% during pregnancy, with partial recovery postpartum. The changes occurred across 94% of the brain’s gray-matter volume and were especially prominent in regions associated with social cognition.
According to UT Southwestern Medical Center, "pregnancy brain" refers to a pregnant woman’s forgetfulness during and shortly after pregnancy. This can range from forgetting why you went into a room, putting objects in strange places, forgetting to do things, and general absentmindedness.
The Source: The information for this story was provided by the study "High-level estrogen impairs memory via estrogen receptor signaling in a hypothalamic–hippocampal neural circuit" published on Aug. 25, 2026. Comments from researcher Zheng Sun were included. Information from other studies, UT Southwestern Medical Center and Reuters was also used. This story was reported from Los Angeles.