You have a million and one things to think about during pregnancy: eating well, exercising, cutting out bad habits. The idea that your blood may not match your baby's might not be at the top of your list of concerns. But it should be an issue to consider. If you are Rh negative and your baby is Rh positive, the mismatch could have serious consequences.
It's usually not a problem with a first pregnancy. But it could present trouble for future babies who are Rh positive--the mismatch is a problem only when the mother is Rh negative and the baby is Rh positive. The antibodies may try to destroy the blood of the future babies, and babies born with this condition may have to have their blood replaced.
A simple blood test, performed very early in pregnancy shows, if you're Rh negative. If you are, a second blood test near week 28 will check whether your blood contains Rh antibodies.
If the second test doesn't show any antibodies, you'll be treated with an injection of Rho(D) globulin (most women get injections of RhoGam) at about week 28 of pregnancy. The injection prevents antibodies from forming. You'll also be treated any time there's a chance that fetal blood has mixed with yours (after amniocentesis, for example). After birth, your baby's blood will be tested. If it's Rh positive, you'll be given Rho(D) globulin again within 72 hours, which should protect future babies. If it's Rh negative, you won't need another injection until your next pregnancy.
Ask your health-care provider any questions you might have about Rh incompatibility.