They say being a parent is like having your heart walk outside your body. While it may feel true that your baby is now part of you in a figurative sense, it turns out there’s science to suggest that’s literally the case. During pregnancy, fetal cells can pass through the placenta into the mother’s bloodstream and take up residence—a phenomenon called microchimerism.

A mother passes nutrients, air, water and other vital materials to the baby via the placenta during pregnancy. In return, the baby gives mama a tiny present that lasts for years, decades or even her lifetime. You may carry cells from your children and your mother simultaneously—highlighting the deep connection between parent, child and even between generations.

What is microchimerism?

“Microchimerism is a fascinating and complex phenomenon in which a small number of cells from one individual exist within the body of another genetically distinct individual,” explains reproductive scientist Renu Bala, PhD. “This can happen through several mechanisms, such as pregnancy, blood transfusions or organ transplants.” Fetal microchimerism (FMc) specifically refers to the movement of cells from the fetus to the mother’s body via the placenta.

Dr. Bala explains that there are several reasons this can happen. “First, the placenta forms a physical link between maternal and fetal circulatory systems,” she shares with Motherly. She also explains that mama and baby may express special molecules that help cells bind or move around more easily. Also, your immune system is lower than usual while pregnant, which may temporarily allow cells to enter your bloodstream without being attacked.

The transfer of cells starts early—showing up in the blood at 4 or 5 weeks of pregnancy. These cells are also unique because they can grow into different types of tissue all over the body (called pluripotent). Fetal microchimerism cells have been found in maternal organs like the heart, lungs, brain, breast and skin.

Cells from pregnancy can exist for decades

How long the cells from your baby actually hang around in your body is up for debate. “In some instances, these cells have been detected decades after the pregnancy, suggesting a long-term or potentially permanent residency,” shares Dr. Bala. “In most cases, they diminish over time, particularly if the maternal immune system identifies and targets them for removal.” 

She explains that the timing depends on your body and how your immune system responds after pregnancy. In some cases, fetal cells have been found 30 years after pregnancy and even for a lifetime. This could mean you carry cells from every pregnancy, holding a piece of your babies with you for life.

Do baby’s cells stay with you if you have a miscarriage?

Since fetal transfer happens early in pregnancy, mamas who have experienced pregnancy loss may still have their baby’s cells in their body. “The exchange of cells between the mother and fetus commences relatively early in gestation and does not necessitate a full-term pregnancy for FMc to take place,” says Dr. Bala. “Even in pregnancies that last just a few weeks, there’s the potential for cellular transfer facilitated by the placenta.”

How does microchimerism affect surrogate pregnancies?

Gestational surrogacy means the pregnant mama carries a baby for another family through in vitro fertilization (IVF). There’s no genetic link between the pregnant person and the embryo because the egg and sperm come from the intended parents or a donor, but microchimerism still occurs. “Even though the baby isn’t genetically hers, the surrogate can carry these fetal cells, which can become pluripotent and can become part of various tissue types, in organs like her heart, lungs, brain, breast, and skin for years,” says Dr. Bala.

The health implications for mama are complicated

Fetal microchimerism cells may help and even be protective by helping repair tissues from labor and delivery. “Many studies suggest that fetal cells might travel to maternal wound sites to help with healing by differentiating into cells of different lineages,” explains Dr. Bala.

At the same time, there may be health risks associated with fetal microchimerism that are still being studied. “While generally considered a benign event, microchimerism has been implicated in certain medical conditions. For example, some research suggests it may play a role in maternal autoimmune diseases or certain cancers.” She also shares that the health implications appear to be the same for surrogate pregnancies despite no genetic link.

These cells may help mama and baby’s individual immune systems tolerate each other during pregnancy. “For a long time, we’ve known that these special cells can contribute to autoimmune diseases in pregnant women from previous pregnancies; however, they also play a vital role in helping the immune systems of both mother and baby tolerate each other,” says Dr. Bala. The presence of these cells may even partly explain why certain autoimmune diseases like rheumatoid arthritis calm down during pregnancy.

Microchimerism means we are more connected than we think

While we don’t know exactly what role fetal microchimerism plays in human development and health, we can safely say that it’s a powerful metaphor for the mama-child bond. “Whether it’s through pregnancy or medical interventions like blood transfusion and organ transplantation, the exchange of cells between individuals serves as a testament to the interconnectedness of human biology,” shares Dr. Bala.

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Renu Bala, PhD, is a reproductive researcher and contributor to Genes Wellness.

A version of this story was originally published on Sept. 11, 2023. It has been updated.