Isn't it well established that pregnant women host new brain cells that migrate from the fetus?
https://pmc.ncbi.nlm.nih.gov/articles/PMC2633676/ (2007)
https://academic.oup.com/clinchem/article/67/2/351/6071463?l... (2020)
https://en.wikipedia.org/wiki/Microchimerism#Fetomaternal_mi...
IIR, substantial and sustained fetal cell presence in the mother is often a sign that things are going wrong.