Nevertheless, foreign DNA persisted


A few months ago, an article surfaced and claimed that a woman contains DNA from every sexual partner she’s ever had. Surely unprotected sex can have legitimate consequences, such as unexpected pregnancies and/or risking numerous sexually transmitted infections. But, should females also fear their partners’ DNA persisting inside of them for the rest of their lives? Science says not really…

(Contraceptives can mitigate almost all of these risks.)

We tracked down the original scientific publication that inspired this claim and spoke with the head researcher, Dr. J. Lee Nelson, a member of Fred Hutch Cancer Research Center and Professor of Medicine at the University of Washington. The study, titled “Male microchimerism in women without sons: Quantitative assessment and correlation with pregnancy history,” aimed to assess how frequently microchimerism occurs and whether or not a woman’s pregnancy history can influence her likelihood of being a microchimera.

But what even is a microchimera? And what does it have to do with sex?

A chimera is an organism (ex. human, cat, etc.) that is made from cells that have different DNA, or genetic code. Usually, in organisms that have a lot of cells, all of the cells contain the same DNA. However, in chimeras, this is not true. Chimerism happens when two genetically distinct organisms merge—like a fetus absorbing its twin in the womb. Chimerism can lead to some interesting drama, such as “How a Man’s Unborn Twin Fathered His Child”. By definition, a microchimera is a little bit (“micro”) of a chimera. Indeed, microchimeras contain only a small amount of foreign DNA.

Note: a Chimera is a hybrid fire-breathing monster that comes from Greek mythology.


Microchimeras are real and definitely not monsters.

To be a microchimera, you don’t need to absorb your twin! If you have ever received a blood transfusion or a bone marrow transplant, you were a microchimera for at least a moment in time. If you have ever donated blood, your DNA may be coursing through someone else’s veins, saving another person’s life and giving them the privileged title of “microchimera.”

Females can become microchimeras through pregnancy. In pregnant females, fetal material can cross the placenta and circulate through the mother. Interestingly, this form of microchimerism can last (even decades!) after the pregnancy has ended. This is where the confusion about having your partner’s DNA in you forever originated.

In this study, a team of scientists tested for male DNA in the blood of 120 women—some with rheumatoid arthritis (RA) and some healthy. RA is an autoimmune disorder in which the immune system of a person with RA mistakenly assumes the person’s joint lining is a foreign invader and launches an attack against it. The researchers kept track of the women’s pregnancy histories, including whether they had ever been pregnant, had an abortion, had a miscarriage, or given birth. Researchers found that pregnancies can have a beneficial effect on RA: women who have given birth are less likely to develop RA, and arthritis symptoms can subside in pregnant women with RA. Researchers think this may occur because fetal cells and/or fetal DNA circulating in the mother’s bloodstream could temporarily trick the immune system into working properly again. It is a compelling hypothesis (with more work to be done) that will hopefully help those suffering from RA.

Dr. Lee and her colleagues did not find a difference between women with and without RA in terms of how often male DNA was found in their blood. They did find male DNA in 21% of women who had never given birth to a son. This means that a successful birth is not a requirement to becoming a microchimera.  Pregnancy history matters though—women who had elected abortions in their first trimester were the most likely group to have male DNA floating around in their blood, even when compared to women who had spontaneous abortions.

However, all scientific studies have limitations. In this study, there are two main limitations: 1) small sample size and 2) looking for male DNA, not foreign cells.

The study only looked at 120 participants to determine how common male microchimerism is in women. This number may sound like a lot of people, but in terms of making conclusions about the general population, it is fairly small. To make bold claims about the frequency of something in the population, you must look at thousands of people with a wide variety of age, ethnicity, medical history, and more. That does not mean this study drew irrelevant conclusions, it just means that more studies need to be done to support their work if claims are to be made about the entire human population.

Secondly, the study used a technique called PCR, or polymerase chain reaction, to test whether male DNA was present in the blood. PCR works by finding a specific gene or gene segment, and then making millions of copies of that segment. PCR is useful for figuring out how much of a certain gene is present. In this study, Dr. Lee’s team amplified a male gene (Y chromosome) with PCR to see how much of it was present in female blood.

Their PCR experiment could not find foreign female DNA. If women in the study were ever pregnant with a girl, this form of microchimerism was not detected. Looking for a male gene is a lot easier than looking for a foreign female gene, so most microchimerism studies focus on finding male microchimerism.

Additionally, this means that the study did not look for foreign cells, but only for foreign DNA. The idea that male DNA could be free-floating in the bloodstream cannot be excluded, meaning that fetal cells are not the only potential source of male DNA.

In their conclusions, Dr. Lee and her team explain that male microchimerism is not rare in women without sons. Some of the reasons for this include unrecognized spontaneous abortion, vanished male twin, and DNA of mother’s older brother transferred from the maternal circulation during her own fetal life. Dr. Lee explains that her team lists sexual intercourse as a potential cause for microchimerism because they are “just acknowledging a possibility brought up by others that there could be transient male DNA following intercourse…the thinking is simply that there is male DNA in sperm and that it may take a while to clear this.” To their knowledge, sexual intercourse has never been shown to be a cause of microchimerism.

Male DNA can actually circulate in female blood! Microchimerism can be linked to pregnancy, and thus intercourse. Although microchimerism has not been proven to result from recreational sexual intercourse, scientists will not rule this hypothesis out until they can accurately test it. And if it is proven true that DNA from your sexual partners may circulate in you, should we be scared? I don’t think so.

Copulation, the type of sexual reproduction humans and many other animals engage in, has been around for hundreds of thousands of years. The threat of male DNA in females, presumably, has not changed much since the dawn of homo sapiens. And there is no need to start worrying about it now.

So, why did these scientists even study male microchimerism? Well, before this paper, most microchimerism work only studied women who had given birth to a son. Dr. Lee and her group have shown that a successful birth is not a requirement to be a microchimera. Indeed, they found that male DNA was more likely to be found in a female’s blood if the pregnancy didn’t go to term, because of elected or spontaneous abortion. They also wanted to know if women with RA were different than healthy women in terms of microchimerism but found no difference.

New ways of becoming a microchimera are being discovered, too. A French group found male cells in female livers from fetal life to adulthood (Guettier). Guettier’s team indicated that alternative sources of male DNA in a female can be from a vanished twin, an older male sibling, or a prior miscarriage. In reference to this study, Dr. Lee notes that “there are obviously a number of different ways male cells can be acquired by a female.”

In my humble opinion, microchimerism is flat-out fascinating. It is not that rare for a woman to be a microchimera. And to my knowledge, there are no negative health consequences associated with it. Instead, it is being studied because of its potential to alleviate symptoms of rheumatoid arthritis and other autoimmune disorders. So could retaining foreign DNA in your circulation potentially be an asset or just a simple process of evolution? Hopefully future studies on microchimerism will unravel its purpose.



Guettier C, Sebagh M, Buard J, et al. Male cell microchimerism in normal and diseased female livers from fetal life to adulthood. Hepatology. 2005;42(1):35-43. doi:10.1002/hep.20761.

Landy H, Keith L. The vanishing twin: a review. Human Reproduction Update. 1998;4(2):177-183. doi:10.1093/humupd/4.2.177.

Utter GH, Reed WF, Lee T-H, Busch MP. Transfusion-associated microchimerism. Vox Sanguinis. 2007;93(3):188-195. doi:10.1111/j.1423-0410.2007.00954.x.

Yan Z, Lambert NC, Guthrie KA, et al. Male microchimerism in women without sons: Quantitative assessment and correlation with pregnancy history. The American Journal of Medicine. 2005;118(8):899-906. doi:10.1016/j.amjmed.2005.03.037.



Lead author and expert contact: Sam Tucci

Background research: Maria Paz Prada

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