How a Baby Becomes Part of a Mother

Fetal cells that migrate from the fetus to the mother deliver benefits to both

In the early weeks of pregnancy, a connection is made between the mother and her growing baby as they exchange cells across the placenta. This little-known phenomenon will affect both of them for decades to come.

Microchimerism is when cells from one individual persist in another. The term means a “small chimera”; in Greek mythology, the Chimera is a creature whose body is composed of different animals. Pregnancy is the primary cause of natural microchimerism.

Fetal microchimerism—the transfer of fetal cells to the mother—begins in the first month after conception, often before a woman knows she is pregnant. The mother’s immune system removes some of these cells, but those that persist are absorbed into her body and will remain with her for decades, or possibly for life.

The study of microchimerism is still in its infancy, but we know that fetal microchimerism benefits the baby and mother alike.

Benefits to the Baby

During pregnancy and the early postpartum period, fetal cells seem to act as messengers, triggering changes that improve the chances for the baby’s survival by making the mother’s body more hospitable.

One of the first tasks of fetal cells is to secure a source of nutrition. Cells in the blastocyst (the mass of cells that becomes the embryo) initiate the development of the placenta. Then, even before the placenta has completely formed, fetal cells begin migrating into the mother.

Once in the mother’s body, the fetal cells are linked to a cascade of changes. “Fetal cells that migrate to the mother’s thyroid gland help to increase her body temperature, which is important for keeping her baby warm both in utero and after birth,” says Dr. Jessica Madden, a pediatrician, neonatologist, and medical director at Aeroflow Breastpumps. “Fetal cells that migrate to the mother’s breast tissue help the breasts to produce more milk.”

Besides priming the mother’s body to be an ideal environment, the fetal cells cross the blood-brain barrier, where they appear to help stimulate areas of the brain responsible for maternal attention and emotional bonding.

But what about the fetal cells that remain in the mother long after the baby has outgrown their dependence on her?

Benefits to the Mother

Just as microchimeric fetal cells trigger changes that help the baby, they also play a guardian-angel role within the mother, assisting and protecting the mother for many years.

Fetal cells that remain in a mother’s body appear to play a role in replacing injured cells, Madden explains; for example, they are often found in C-section scars. Amazingly, the skin of a fetus heals completely with no scarring. The cells capable of this incredible feat actively migrate to the site of injury in the mother and differentiate to become skin cells that produce collagen. In this way, they take an active part in repairing the C-section incision from their birth.

This “gift” from the baby also appears to provide a surveillance system for malignant cells, and is associated with protection from breast cancer.

The ability of fetal cells to become whatever is needed to heal the body has been suggested as an explanation for why women have better outcomes in terms of health and longevity. “Research has shown that fetal cells can repair damaged liver, brain, and heart tissue in the mother. And it has been postulated that microchimerism plays a role in women’s lower risk of death from heart disease,” says Madden. They have even been found in appendix tissue, possibly attempting to fix a problem before it becomes dangerous.

Fetal cells migrate into the mother starting in the early weeks of pregnancy and the flow increases as the pregnancy progresses. Interestingly, years later, the largest number of fetal cells in mothers seem to be from pregnancies that ended early. This could be a comforting thought to women who have had pregnancy loss.

“Having ever been pregnant might literally extend or save a woman’s life,” says Madden. “Microchimerism is scientific proof that a mother’s children are always a part of her.”

Shannon Jordan is an author and writer based in Carrboro, N.C. She earned her midwife license in Taos, N.M. and a degree in biology at the University of North Carolina at Chapel Hill. She is the blessed mother of four through birth and adoption.