Does an unborn child and the mother's blood ever mix? Why or why not?
Usually a mother and baby's blood do not mix while the baby is in the womb. The mother's blood runs alongside the placenta, and the nutrients needed by the baby are absorbed and transferred to him/her. A membrane separates baby's blood and mother's blood – all the baby's blood is contained within the baby and placenta.
Sometimes a traumatic event like a car accident, a CVS or amniocentisis procedure, etc. can cause the blood to mix. It also sometimes mixes during childbirth. This usually happens during a traumatic or hard birth, but has been known to happen even during gentle births.