Could unexplained miscarriages be rooted in the brain?

Few humans experience more psychological trauma than women who endure repeated miscarriages. A team of 20 researchers at Israel’s Weizmann Institute of Science sought to answer the question so often asked: Why?

The researchers wanted to determine whether miscarriage survivors show any olfactory differences from a control group of women. They knew to analyze scent processing because many mammals’ olfactory systems are intertwined with reproduction. For example, pregnant mice will miscarry when exposed to the scent of an unfamiliar male who did not father the pregnancy. This is called the Bruce effect—some theorize that it is a response to the availability of a more fit male.

The researchers’ hunch played out. They found that when presented with the t-shirts of men, most of the 33 women who had experienced repeat miscarriages could identify their husbands’ shirts, while most of the control group could not. The differences between the groups were striking. When women did not know what odors they would be smelling, “several of these women said, ‘Oh, my husband’s in here,’” said the study’s coauthor Reut Weissgross in a news release. This happened zero times among the 33 control women.

Brain imaging found corresponding structural differences: The survivors of unexplained miscarriages had smaller olfactory bulbs, which are the switchboards for incoming smells, yet their hypothalamuses showed more response to men’s body odor. When asked to rate men’s body odors on a number of scales, they described the smells quite differently from the control group, indicating that they might experience male body odor differently. These special olfactory abilities mostly did not extend to other types of smells.

The researchers emphasize that this study does not suggest that olfactory systems or body odors cause unexplained repeat miscarriages; the study only determines that a group of miscarriage sufferers have consistent, unique olfactory attributes, and that future miscarriage researchers would be smart to point their attentions toward the brain, not the uterus.