From the fifth month onward, the placental barrier becomes thinner, in pregnancy and fetal cells enter the mother, and maternal cells enter the fetus. So, your mother could have passed on to you during your gestation her cells, cells from your older siblings, or even cells she retained from your maternal grandmother.

Medicine student, profesional musician and gymbro :)

That's awesome. There are downsides to cells crossing the barrier too such as Rhesus disease but a lot of these cases can be treated with an injection called anti-d.

That's awesome. There are downsides to cells crossing the barrier too such as Rhesus disease but a lot of these cases can be treated with an injection called anti-d. 

https://www.nhs.uk/conditions/rhesus-disease/