While it's been known for a while that fetal cells migrate into a mother's body during pregnancy, it hasn't really been understood what types of cells stick around and what they do. But a new mice study conducted by researchers at Tufts University might help explain what goes on inside the body when a woman gets knocked up.

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For the study, researchers used a male mouse that had two copies of a green fluorescent protein gene and mated him with a normal female, resulting in every fertilized fetus carrying the fluorescent protein gene. When its cells made it into the mother, the researchers could then recognize the fluorescent signal and distinguish them from the mother's own cells.

Interestingly, the researchers tracked down these glow-in-the-dark cells from the placenta and the fetus all the way inside of a pregnant mouse's lungs. They were able to determine that some of the cells were immune cells, while others were undefined connective tissue. It's not 100% clear what these migrating cells where up to. Leading researcher Diana Bianchi explains (via Live Science):
We and other people have shown they stay around for decades. They aren't the original cells so there must be some way they can give rise to daughter cells in the mother's body. We strongly believe that there are implications for the future health of women who are or have previously been pregnant.
It's not totally clear in the first place how the cells even get across the placenta and into the mother's body, but researchers speculate that the are leaky spots the cells that form the barrier in the placenta that keeps the baby's blood separate from the mom's blood.