Most people seem to have cells that crossed the placenta from their mother into their developing brain in utero, even if they're now in their 80s https://www.newscientist.com/article/2593373-cells-from-your-mother-could-stay-in-your-brain-your-whole-life/?utm_term=Autofeed&utm_campaign=echobox&utm_medium=social&utm_source=Twitter#Echobox=1791995228