As far as life events go, few affect a woman as much as the emotional and hormonal roller coaster of a nine-month pregnancy and the final climactic birth of a little bundle of joy.

It seems logical that once baby is born, the hormonal shifts and biological mayhem wreaked on Mom’s body would settle down and, with the exception of a few stretch marks and extra pounds, things would return to normal.

Scientists who study fetal microchimerism say that nothing could be further from the truth.

fetal cells

During pregnancy, cells from the fetus hitch a ride through the placenta into the maternal bloodstream and can be dropped off in virtually any type of maternal cell — lung tissue, brain tissue, heart muscle, liver cells — and may remain there for decades, integrating themselves into everyday cellular processes.

This is also true of women who have not completed pregnancies, although the further into gestation a pregnancy is carried, the more fetal cells are transported.

In some cases, this integration is beneficial and protective, possibly even against cancer. In others, it can be linked to harmful outcomes and may even be implicated in some autoimmune diseases.

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What Exactly Is Fetal Microchimerism?

The integration of fetal cells into the mother’s body — fetal microchimerism — is a topic Amy Boddy, Ph.D., a researcher at Arizona State University's Department of Psychology who specializes in evolutionary biology and genetics, has studied for years.

The term is derived from the hybrid beasts of Greek mythology that were depicted with body parts of many different animals — the head of a lion, body of a goat, and tail of a serpent, for example.

The scientific definition differs, but the concept persists.

“Chimera is the mixing of cells from two genetically distinct individuals. Microchimerism is a small-scale version of this — a small population of cells with a different genetic background within an individual,” Boddy told Healthline via email.

Working with a team of scientists, Boddy recently tried to make sense of the many studies of fetal microchimerism, looking for patterns or clues as to why in some cases fetal cells benefit the mother but are detrimental in others.

“Some of the fetal cells that are transferred across the placenta have stem-like properties. That is, they have the potential to specialize into many different cell types… depending on where they land in the mother’s body.” Boddy explains. “Whether a fetal cell has a beneficial or detrimental effect partially depends on what kind of cell it develops into, but moreso on how the mother’s body responds to having those extra cells around.”

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Survivor: Mom vs. Baby Edition

Boddy’s article in the Aug. 28 online issue of BioEssays and a user-friendly online video detail the team’s findings.

The research attempts to explain the paradoxical maternal response to fetal cells using what’s known in evolutionary biology as cooperation and conflict theory.

An allele is an inherited piece of genetic material that determines how a gene is expressed — think blue eyes or brown eyes.

Essentially, the theory suggests that alleles for cooperation or competition will spread through a population depending on how they benefit the individual’s ability to survive and reproduce.

If cooperation increases the individual’s access to resources, those alleles will endure. But if resources are scarce, selfish alleles will win out.

“When individuals have mutual interests, as well as selfish interests, both alleles for both cooperation and conflict can work together to increase overall survival,” Boddy says.

This is complicated in the case of fetal microchimerism.

“It is in both the mother’s interest and the offspring’s interest for the offspring to survive to reproduce itself,” Boddy states. “But the mother may also be interested in future offspring, so may want to save some resources, while the offspring may only be interested in its own survival, leading to conflict when the offspring requests too many resources.”

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Good News, Bad News: How Fetal Cells Affect Mom

In keeping with cooperation and conflict theory, fetal cells play different roles in different maternal tissues.

And, according to the many conflicting studies on microchimerism, they may be associated with protecting against some diseases while contributing to the development of others.

If hitchhiking fetal cells end up in the brain, for example, they may be protective against Alzheimer’s but predictive of Parkinson’s.

Fetal cells in breast tissue? Some types of breast cancer are less likely, but others may be more so.

One promising area of research is that of autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, and disorders of the thyroid.

While much is still unknown about how fetal cells contribute to a mother’s autoimmune response, researchers think that competition for resources triggers a maternal immune response, causing the body to attack itself indiscriminately because it doesn’t recognize the fetal cells as foreign.

Read More: Mother’s Weak Thyroid Increases Baby’s Chance of Autism »

What’s Next for the Chimeras?

While it’s hard to determine exactly where Boddy and her colleagues’ research will lead, there are many potential benefits to learning more about the role of fetal cells in maternal tissue.

Looking at the presence or absence of fetal cells could help develop more sensitive tests for birth defects in pregnant women — and, if long-term research studies are conducted, science might find a way to use fetal cells to prevent certain types of breast cancer or increase lactation in breastfeeding mothers.

“Currently the field is painting a paradoxical picture of fetal cells… Whether fetal cells are helpful, harmful, or just bystanders is dependent on environmental conditions — and in this case, the environmental conditions [are] the mother’s body,” Boddy said. “It would be really cool to understand functionally what the fetal cells may be doing in a normal healthy pregnancy and postpartum. Future research on these topics could have huge implication for mother’s health and wellbeing after pregnancy.”