Movement of stem cells from mother to fetus
Since the maternal cells can be found in a number of tissues including 


  In second trimester fetuses the thymus, lung, heart, pancreas, liver, spleen, kidney, adrenal gland, ovary, testis and brain had maternal Mc (microchimerism)


it's clear that these are stem or stem like cells that are being incorporated into fetal tissue.

Since these cells arrive while the immune system is developing, they are considered as self and not foreign. But a breakdown of that immune tolerance is thought to be involved in a number of autoimmune diseases, especially systemic sclerosis which resembles a host vs graft disease where the patient's immune system attacks tissues holding the maternal cells.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3516290/