And so are you!

When you were a fetus, you were sending millions of your cells through the placenta into your mom. And she was sending her cells into you, although to a lesser degree. These cells made themselves right at home, differentiating into heart, blood, and even brain cells. This phenomenon is called feto-maternal microchimerism, and is one of the wildest things in placental mammal pregnancy.

What?

Microchimerism is generally defined as the presence of a small population of genetically distinct cells in an organism. When the fetus sends cells to the mother it's called Fetal Microchimerism (FMc) and when the mother sends cells to the fetus it's called Maternal Microchimerism (MMc). The actual cells are fetal microchimeric cells (FMC) and maternal microchimeric cells (MMC).

yes somehow this is the official pubmed diagram

You may have heard that the placenta is an organ that provides oxygen and nutrients to the fetus during development. Which is true. What's less commonly known is that the placenta also does bidirectional cell and genetic material trafficking, similar to drugs and humans across the US-Mexico border. Most of these cells are quickly killed by the mother's immune system immediately after to two weeks [...]