|probably the most complimentary thing I have been compared to.|
"you are going to have a boy/girl"
"you are carrying high/low"
"you look like an olive on a toothpick/beached whale"
"you probably have some of your husband's DNA/baby's cells in your brain now."
That last one requires a little more explanation. How could new external foreign cells get into my brain? First of all there is the blood-brain barrier which prevents your own blood cells from getting mixed in with your neurons, and second of all there is the placental barrier that prevents your blood from mixing with the baby's blood.
Neither of these barriers are perfect. Certain drugs and chemicals can cross the blood-brain barrier, and drugs and chemicals that a pregnant woman ingests can cross the placental barrier to get to the baby. But are these barriers so leaky that whole cells can get through?
Apparently they are. Dawe et al., 2007 explains possible ways that this can happen.
|The placenta, up close. (Dawe et al,. 2007 Figure 1)|
"The mechanism by which cells are exchanged across the placental barrier is unclear. Possible explanations include deportation of trophoblasts, microtraumatic rupture of the placental blood channels or that specific cell types are capable of adhesion to the trophoblasts of the walls of the fetal blood channels and migration through the placental barrier created by the trophoblasts." (Dawe et al., 2007)
It is also not clear how these baby cells, once in the mother, could cross the blood-brain barrier. In fact, it is not perfectly clear (as of this 2007 paper) that these cells do get into the mother's brain in humans, though studies have shown fetal DNA-containing cells in the brains of mice.
So in conclusion, if you have ever been pregnant, you probably still have some of that baby's DNA (and consequently some of the baby's father's DNA) in your body. If you were pregnant with a boy, then you probably have Y chromosomes in some of your cells! It even seems that mothers can transfer cells from previous babies into future babies. This means that if you have an older brother or sister, you might have some of their DNA in your body as well.
The next question is: Do these foreign DNA cells have a meaningful impact on your body?
Dawe GS, Tan XW, & Xiao ZC (2007). Cell migration from baby to mother. Cell adhesion & migration, 1 (1), 19-27 PMID: 19262088