Scientists don’t know for certain what biological mechanisms cause these findings, but past research suggests microchimerism may boost immune surveillance—that is, the body’s ability to recognize and destroy pathogens and cells that might become cancerous—and also play a role in the repair of damaged tissue, helping form new blood vessels to heal wounds. Microchimerism is also associated with a lower risk of Alzheimer’s disease and breast cancer.

These fetal cells migrate all over a mother’s body, becoming part of the heart, the brain, and blood—and fascinating scientist and artists alike. Writer Sarah Gerkensmeyer, mother of two sons, says she likes to think that they never completely left her, and that microchimerism from her younger son Charlie, born in 2011, helped give rise to her short story collection.

“When I was pregnant with him, those weird stories were in me, washing around and fusing together right alongside my developing son,” she wrote last year. “It’s a nice, strange thought, isn’t it? That he gave me those stories to tell.”

Some journalists have asked if the presence of male cells can cause women to think differently, said Mads Kamper-Jørgensen, an associate professor of public health at the University of Copenhagen and the lead author of the International Journal of Epidemiology study.

The answer: No. He also noted that the health benefits of microchimerism are the same whether the cells are male or female. “Sons aren’t any better than daughters.”

The reason why much of the research has focused on microchimerism stemming from males, he explained, is because of the ease in measurement. In blood samples, male Y chromosomes stand out among a woman’s XX. By comparison, finding a daughter’s genetically distinct cells in a mother is expensive and difficult because the chromosomes are all XX. Detecting microchimerism in men is also a challenge, because the female X chromosomes are hard to differentiate from the male XY. (Microchimerism is probably more frequent in women, because pregnancy is a natural avenue for transferring cells, but individuals may also pick up genetically distinct DNA after an organ transplant or transfusions, or in utero if they had a twin.)

A 2004 study found the presence of male genes in 21 percent of women overall—even among those who had only given birth to daughters, had a miscarriage, underwent an abortion, or had never been pregnant. Researchers speculate the unknown DNA could have come from a miscarriage these women never recognized, or from an older brother who transferred cells to their mother, who in turn passed the genes onto subsequent children. Or—here’s where the science starts to feel like sci-fi—women could have picked it up through sexual intercourse, traces of past lovers never lost.

Though the idea is intriguing, it’s also “sort of impossible” to study, Kamper-Jørgensen said, because of the intimate details participants would have to agree to share, and the testing that would have to begin even before women have sexual intercourse and after each new partner.