If an Rh-​negative person is exposed to the blood of an Rh-​positive person, he or she may form molecules called antibodies that destroy Rh-​positive blood.

This is really a common, normal response of the body’s immune system as it fights off foreign invaders. The problem is, an Rh-​negative mother may have a baby who inherits Rh-​positive blood from its father. This presents no problem when the placenta works perfectly and the baby’s blood is kept separate from the mother’s. (Compare Psalm 139:13.) But because our bodies are imperfect, a small amount of the baby’s blood may sometimes leak through and come into contact with the mother’s. Occasionally, this happens because of some medical procedure, such as an amniocentesis (drawing a sample of the fluid in the birth sac surrounding the developing baby).

Can laboratory testing tell me if my developing baby is at risk?

Yes, to an extent. Antibody levels in the mother’s blood can be measured during pregnancy to tell if she is producing antibodies against the baby’s blood. Also, amniocentesis can help tell if the baby’s blood is being destroyed and the baby is in danger. Yet, amniocentesis sometimes presents its own complications, and so there ought to be caution about undergoing it.

According to doctors, how often should I take the shot?

Authorities say that the shot should be given soon after any event that may have caused Rh-​positive blood to enter the bloodstream of an Rh-​negative woman. Thus, current recommendations are that the shot be given within 72 hours of delivery of the baby if the baby’s blood is found to be Rh-​positive. The same recommendation holds for an amniocentesis or a miscarriage.