At your first prenatal appointment, your OB will take blood for a long list of screenings. One of them will be to determine your blood type. You may already know if you’re type A or O, but what’s important during pregnancy is whether or not you’re “positive” or ”negative.” Your OB will test your blood for Rh, a protein present in about 85% of the population. If you’re Rh-negative and the father is Rh-positive, the fetus can inherit the Rh factor from the father. This makes the fetus Rh-positive too. Problems can arise when the fetus's blood has the Rh factor and the mother's blood does not. So, baby is “positive” but you are “negative”. If this happens, it’s possible for you to develop antibodies to your baby, in essence your body will think it’s allergic to baby. To prevent any complications, you’ll need injections of a medication called RhoGAM at 28 or 29 weeks and within 72 hours after delivery to prevent problems.
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