It’s common knowledge that mothers pass nutrition, DNA, maternal cells, and immunity, among many other things, to their babies when they are in the womb. And as mothers often say, their babies are always a part of them. While many mothers say this figuratively, scientists have discovered that if you’ve physically given birth, it’s literally true as well: Some of your baby’s DNA will always be present in your body, even after delivery.

When a baby is in the womb, a part of their DNA moves to the mother’s body through the placenta and embeds itself in the mother’s tissues, becoming a permanent part of her. This is why mothers are made from not only their own genetic material, but also the genetic material of their children. Scientists call this occurrence microchimerism.

This phenomenon has lifelong consequences on the mother’s body and health. Read on to learn how and why it occurs.

What is fetal microchimerism?

Fetal microchimerism is a biological phenomenon in which a small number of fetal cells cross the placenta during pregnancy and remain in the mother’s body. These cells can circulate in the mother’s blood and tissues for years or even decades after pregnancy.

The term Microchimerism is formed from ‘micro,’ which here refers to the fact that the amount of cells exchanged is tiny, and ‘chimera,’ a creature from Greek mythology that was part lion, part goat, and part serpent. Microchimerism means the presence of one or more genetically different cells in an organism.

We’ve known about the concept of passing cells from one body to another for more than a century now. In the womb, the mother’s cells pass to the baby through the placenta. This phenomenon is called maternal cell microchimerism, mother fetal microchimerism or maternal-fetal transfer. Identical twins share a placenta and are also known to sometimes exchange microchimeric cells through it.

Interestingly, this transfer of cells is not unidirectional, that is, just as mother’s cell pass to the baby, fetal cells enter through the placenta and are transferred to the mother’s tissues and organs. This passage of cells from the fetus to the mother is called fetal cell microchimerism, fetal maternal microchimerism or fetal-maternal transfer. When the baby’s cells from different pregnancies become a permanent part of the mother, it’s as though she becomes a chimera of sorts.  

This microchimerism in pregnancy, that is, the transfer of fetal cells, begins in the first trimester, and these fetomaternal microchimeric cells can stay in the blood and tissues for decades. Hence, even when a mother goes through a loss of pregnancy, she still retains her baby’s cells in her body. The baby leaves a long-lasting biological imprint on the mother. 

It was in 1979, for the first time, that researchers at Stanford University found some fetal cells in maternal blood with Y sex chromosomes. Since women only have X chromosomes, the Y chromosomes were found to have come from her son, and transferred to her body during the pregnancy. This discovery marked the onset of further research in fetal microchimerism.

Does the firstborn leave their DNA in the mother?

Yes, the firstborn can leave their DNA in the mother. This phenomenon, known as microchimerism, occurs when cells from the fetus cross the placental barrier and enter the mother’s bloodstream.

Although many of these fetal cells are targeted and destroyed by the mother’s immune system, some manage to survive and integrate into various tissues and organs, becoming a lasting part of the mother. This exchange of cells between mother and fetus means that a mother can carry a small number of cells from her firstborn (and any subsequent children) for years or even decades after pregnancy.

Interestingly, the transfer of cells doesn’t stop with just the mother and the fetus during the current pregnancy, but it is more complex.

According to this report by the European Journal of Obstetrics & Gynecology and Reproductive Biology, a mother’s body can accumulate cells from each baby and transfer cells from the older sibling into the younger one during subsequent pregnancies, thus forming more elaborate microchimeras. This situation came to light when male hepatocytes were found in a little girl’s liver, although she had never received any blood transfusion.

What do the baby’s cells do in the mother’s body?

Once the baby’s cells cross the placenta and pass to the mother’s bloodstream, they circulate in her body and embed themselves in her tissues and organs like the heart, brain, liver, bone marrow, thyroid, and skin. And stay there even after her pregnancy. 

As per the research published in BioEssays, a few interesting facts have come to the forefront regarding the function of fetal microchimerism in the mother’s body. 

1. Fetal microchimeric cells in the breast increase maternal milk production and quality.  It is in the baby’s fitness interest to increase maternal milk production and quality.
   
2. Fetomaternal microchimerism in the thyroid affects metabolism (increases maternal body temperature) and heat transfer to the baby.
   
3. Fetal cells influence neural circuitry, thereby enhancing attachment and bonding mechanisms. Babies are likely to receive more resources, including an abundance of genes regulating oxytocin and prolactin production, as a result of increased maternal attachment and bonding.
   
4. Fetal cells may be important in conception and retention of the fetus. The maternal body must suppress immune function for successful placentation and gestation. Hence, fetal cells could play a role in tolerance for a successful pregnancy.  Fetal cell quantity in the maternal body contributes to signaling pregnancy success, completion, and/or termination.
   
5. Fetal cells do not serve as one-time pregnancy gifts but play an active role in helping mothers remember previous pregnancies so that their immune systems can tolerate a fetus with foreign traits derived from their father, says this research led by experts at Cincinnati Children’s.

If we keep evolutionary reasoning in mind, this could be a mechanism by which the fetus ensures maternal fitness to enhance its own chances of survival.

How does fetal microchimerism affect the mother’s health?

The presence of fetal cells has been associated with both positive and negative effects on maternal health, states a leading research paper on fetal microchimerism and its role in maternal health in the journal BioEssays. While the role of fetal microchimerism in lactation, thyroid function, autoimmune disease, and cancer has been established, fetal microchimerism cells may also play a role in regenerating the maternal organs and restoring normalcy after the physiological and structural changes that occur during gestation.

Fetomaternal microchimerism could bring about diseases of pregnancy, including Alzheimer’s, autoimmune diseases, and might be implicated in some cancers, while some forms of fetal-maternal microchimerism could be linked to repair and regeneration of tissues, defense against cancer and infection, and potentially explain the longer life expectancy of females.

Alzheimer’s

Scientists discovered cells containing the Y chromosome in the brains of deceased women. These cells were found in multiple regions of the brain and in more than 60 percent of brains. Multiple pregnancies have been found common in women with Alzheimer’s disease (AD). Because of this, scientists suspected that there would be a larger number of fetal cells in the brains of women with AD. The results were the opposite. The number of fetal cells in women with AD was much lower and scientists don’t know why.

Cancer

The jury is still out on this one. A study published in the American Association of Pharmaceutical Scientists, on understanding the role of fetal microchimerism in cancer states that “Microchimeric cells have been identified in healing and healed tissues as well as normal and tumor tissues. This has led to the hypothesis that fetal microchimerism may play a protective role in some cancers and may provoke other cancers or autoimmune diseases.”

But the presence of microchimeric cells in tumor tissues also indicates that they might be connected to tumor growth, so more research needs to be done about the connections between microchimerism and cancer.

Rheumatoid arthritis

In a 2010 study published in Arthritis and Rheumatism, scientists found that pregnancy provides vaccine-like protection against arthritis and rheumatism in women. For more than a century, doctors have been aware of the fact that arthritic pain reduces with pregnancy and resurfaces later. 

According to J. Lee Nelson, one of the study’s authors, “Protection starts about a year after birth, and then gradually attenuates after about 15 years.”

Maternal tissue repair

Fetal cells can help in tissue repair. Baby cells are pluripotent like stem cells, meaning they can grow into different kinds of tissues and also help in tissue repair. After these cells embed themselves in the mother’s tissue, they grow into the same surrounding tissue by chemical interaction with the neighboring cells.

Various studies have been made to find microchimeric fetal cells at the sites of maternal tissue injury after a Caesarean section and their contribution to maternal tissue repair.

A bond that doesn’t end at birth

Long after pregnancy ends, traces of your baby remain woven into your body at a cellular level. Fetal microchimerism reminds us that motherhood is biological, enduring, and deeply interconnected.

As science continues to uncover how these shared cells influence healing, immunity, and health, one thing feels certain: the bond between parent and child is not something you leave behind in the delivery room. It lives on, quietly and profoundly, within you.

You may also like:

• The earliest signs of pregnancy, before your test even turns positive
• Why am I snoring during pregnancy? Causes, hormones, and when to call your doctor
• First-trimester exhaustion: Why pregnancy feels like running a marathon

Sources:

1. Maternal blood with Y sex chromosomes. Scientific American. 2010. Beyond Birth: A Child’s Cells May Help or Harm the Mother Long after Delivery.
2. Fetal microchimerism and maternal health. BioEssays. 2015. “Fetal microchimerism and maternal health: A review and evolutionary analysis of cooperation and conflict beyond the womb.”
3. Multiple pregnancies. National Library of Medicine. 2006. “The number of pregnancies is a risk factor for Alzheimer’s disease.”
4. Fetal microchimerism in cancer. National Library of Medicine. 2015. “Fetal Microchimerism in Cancer Protection and Promotion: Current Understanding in Dogs and the Implications for Human Health.”
5. Rheumatoid arthritis. National Library of Medicine. 2010. “Does Pregnancy Provide Vaccine-Like Protection Against Rheumatoid Arthritis?”
6. Cell Adhesion and Migration. National Library of Medicine. 2007. Cell Migration from Baby to Mother. 
7. Microchimerism could be either beneficial or deleterious. European Journal of Obstetrics & Gynecology and Reproductive Biology. 2011. Fetal microchimerism: benevolence or malevolence for the mother?
8. Role of fetal cell microchimerism seems to have a detrimental role in autoimmune diseases. The Journal of Clinical Endocrinology & Metabolism. 2012. Microchimerism and Endocrine Disorders.
9. An evolutionary perspective of FMc. iScience. 2022. Feto-maternal microchimerism: Memories from pregnancy. 
10. Fetal microchimerism. Scientific Reports. 2024. Fetal microchimeric cells influence maternal lung health following term and preterm births.
11. Chimerism. National Library of Medicine. 2014. Microchimeric fetal cells play a role in maternal wound healing after pregnancy.
12. Fetal microchimerism in tissue repair. Developmental Cell. 2022. Fetomaternal microchimerism in tissue repair and tumor development.