A fetus is not part of the mother’s body

Fetal cells may do more than simply migrate to maternal tissues. The authors suggest they can act as a sort of placenta outside the womb, redirecting essential assets from the maternal body to the developing fetus. Cells derived from the fetus — which can persist in maternal tissues for decades after a child is born — have been associated with both protection and increased susceptibility to a range of afflictions, including cancer and autoimmune diseases like rheumatoid arthritis.

But, as co-author Wilson Sayres, cautions, “it’s not only a tug of war between maternal and fetal interests. There is also a mutual desire for the maternal system to survive and provide nutrients and for the fetal system to survive and pass on DNA.” 

If some degree of fetal microchimerism exerts a beneficial effect on maternal and offspring survival, it will likely be selected by evolution as an adaptive strategy.

A review of existing data on fetal microchimerism and health suggests that fetal cells enter a cooperative relationship in some maternal tissues, compete for resources in other tissues and may exist as neutral entities — hitchhikers simply along for the ride. It is likely that fetal cells play each of these roles at various times.

For example, fetal cells may contribute to inflammatory responses and autoimmunity in the mother, when they are recognized as foreign entities by the maternal immune system. This may account in part for higher rates of autoimmunity in women. (Women have three times higher rates of rheumatoid arthritis, compared with men.)

Fetal cells can also provide benefits to mothers, migrating to damaged tissue and repairing it. Their presence in wounds — including cesarean incisions — points to their active participation in healing. In other cases, fetal cells from the placenta are swept through the bloodstream into areas including the lungs, where they may persist merely as bystanders.

Hoekzema and her colleagues set out to change that. Working in Spain, in affiliation with the Autonomous University of Barcelona, they used MRI scanning to examine the brains of 25 women who had never had children, both before they became pregnant and again from 3 weeks to a few months after they gave birth. The team also scanned 19 first-time fathers at the same intervals, 17 men without children, and 20 women without children who did not become pregnant during the study. Then, they used computer-based analyses to measure changes in gray matter volume.

The findings showed highly consistent gray matter volume losses in the mothers and not in the other groups, the team reports today in Nature Neuroscience. The changes occurred primarily in areas of the brain involved in social tasks like reading the desires and intentions of others from their faces and actions. The hippocampus, a region associated with memory, also lost volume. What’s more, the team found that the mothers’ scores on a standard test that gauges the degree of a mom’s attachment to her infant could be predicted to a significant degree based on the changes in their gray matter volume during pregnancy.

The scientists also used MRI scans to watch the women’s brains work in real time, as they looked at photos of their own infants and of other babies. Several of the brain areas that had lost gray matter during pregnancy responded with the strongest neural activity to their own babies as opposed to the photos of other infants.(Comparisons between the brain’s response to photos of a mother’s own infant and to photos of other infants is a common measure researchers use to gauge neural responses to babies.)
Two years later, 11 of the 25 mothers—those who had not become pregnant again—returned for MRI scans. The scans showed that gray matter loss remained—except in the hippocampus, where most volume had been restored. The changes were so consistent that a computer algorithm could predict with 100% accuracy whether a woman had been pregnant from her MRI scan.