A quick note since this has come up a few times.... In theory, the blood in the "placental curcuit" belongs to the baby - the moms blood in the uterine vessels interfaces with the baby's blood through diffusion, etc - but not by direct mixing. However, in practice some maternal fetal transfusion can occur. The test to evaluate this is the KB [Kleihauer-Betke stain] test which looks for maternal cells in the newborn's circulation. The scenario where this is most common is with precipitous delivery, placental abruption, placenta previa, tetanic labor or manual removal of the placenta. It can also occur during version [turning the baby inside the mom - done from the outside] Gail Gail Hertz, MD, IBCLC Pediatric Resident author of the little green breastfeeding book - disclaimer: owner of Pocket Publications