The Rh+ type blood cells may cross placenta directly at any time during pregnancy.
A transitory embryonic and later fetal organ, the placenta starts to form from the blastocyst soon after implantation. It is a crucial endocrine organ that produces hormones that control both the physiology of the mother and the fetus throughout pregnancy. It also plays crucial roles in mediating the passage of nutrients, gases, and waste between the physically separate maternal and fetal circulations. The placenta forms a species-dependent connection to the fetus through the umbilical cord and, on the other side, to the mother's uterus. Although they are present in marsupials and other non-mammals at varied stages of development, placentas are a distinguishing feature of placental mammals. About 150–200 million years ago, mammalian placentas most likely underwent their first evolution.
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