Scientists discover possible cause of 'momnesia' pregnancy forgetfulness

Many pregnant women say they forget where they put their keys, walk into a room and can't remember why, or lose track of what they were saying. This fuzzy feeling is often called "pregnancy brain" or "momnesia." Now, scientists say they have found a biological reason why this happens — and it has to do with how the hormone estrogen affects the brain during pregnancy.

Researchers found that high levels of estrogen, a hormone that rises steadily during pregnancy, change the way two important brain areas talk to each other. One area is called the hypothalamus, which controls many basic body functions. The other is called the hippocampus, which helps people form and store memories. When estrogen levels are high, it triggers certain cells in the hypothalamus to send "quiet down" signals to the hippocampus, making memory worse.

To study this, scientists tested mice that were given extra estrogen to copy what happens during pregnancy. Those mice had trouble remembering things compared to mice with normal estrogen levels. Their mood, however, stayed the same — only their memory was affected. When researchers blocked the pathway between the hypothalamus and hippocampus, the memory problems went away. And when they activated that same pathway without raising estrogen, memory problems appeared anyway.

The memory fog tends to be strongest in the last months of pregnancy and can continue for a while after the baby is born. It affects different kinds of memory, including remembering people's names, recalling where things are, and keeping up with conversations. Dr. Zheng Sun, a professor at Baylor College of Medicine, said this is common but not dangerous. "About 80% of pregnant women notice those changes," he said.

Dr. Sun was careful to explain that being forgetful during pregnancy does not mean someone is becoming less smart overall. "It is not serious or medically disabling, although it does affect daily functioning and confidence," he said. The effects are specific — they target certain memory tasks rather than a person's overall intelligence or ability to think.

A key discovery was a protein called "estrogen receptor alpha." This protein carries estrogen's signals into cells in the hypothalamus. When estrogen is high, this protein causes certain brain cells — called GABAergic neurons — to become too active. These cells normally send signals that slow other brain activity down. When they fire too much, they end up reducing how well the hippocampus works.

To check if the same thing happens in humans, the researchers also studied 70 women at different stages of pregnancy. They compared those women to women who were not pregnant, using memory tests that measured short-term, long-term, and working memory. Women in the later stages of pregnancy did worse on several memory tasks, and those with higher estrogen levels tended to perform worse. This matched what the team had seen in the mice.

Dr. Yanlin He, a researcher at Louisiana State University's Pennington Biomedical Research Center, said the human results support the idea but are not proof on their own. "We did not directly manipulate or measure this specific neural circuit in pregnant women," he said. That means scientists cannot say for certain the same brain pathway causes memory issues in humans the way it does in mice. Still, the connection between estrogen and memory problems was clear in both groups.

The study was published in a journal called Science Bulletin. Scientists hope that understanding what causes pregnancy brain could one day help doctors support pregnant women better. For now, experts want women to know that forgetting things during pregnancy is very common, it is not their fault, and the fog usually clears up after giving birth.