If it is a male fetus, then you just look for the Y-chromosome. Else, you need to do a DNA analysis.

W

if it is a male child look for genes that are on the Y chromosome.

This is called microchimerism, sometimes -if sustained until few months postpartum- detectable by the immune system leading to autoimmune disease.

Detection methods include -beside Y chromosome as said above- HLA typing.

https://onlinelibrary.wiley.com/doi/full/10.1046/j.1537-2995.2001.41010039.x?sid=nlm%3Apubmed