Transplacental passage of foetal blood cells.

With the brightly fluorescent Y chromatin as a marker, the proportion of foetal lymphocytes in maternal blood was found to be 0.02-0.16% in 7 of 11 primigravidae bearing a boy, but not a single XY mitosis was found among 112,000 leucoagglutinin-stimulated mitoses from the same women. The proportion of foetal lymphocytes did not change appreciably during pregnancy (first appearance detected at 15 weeks' gestation), nor were there any unequivocal foetal erythrocytes in spite of ABO and Rh compatibility. These findings suggest that foetal lymphocytes pass actively into the mother's blood relatively early during pregnancy. The cells are protected from immunological elimination, and therefore may remain in the maternal blood for long periods. They are a valuable potential source of material for antenatal foetal diagnosis.