Rh Factor and Pregnancy

What is Rh factor?

If you’ve ever had your blood type tested, you know whether you’re A, B, O, or AB. You also probably know whether you’re Rh positive or negative. The difference between B positive and B negative is a single protein called Rhesus (Rh) factor. If you have the protein sitting on the surface of your red blood cells, you’re positive. If you don’t, you’re negative.

What is Rh incompatibility?

If you’re Rh negative and your baby’s Rh positive, you have Rh incompatibility. As soon as your body comes into contact with the Rh protein — a substance it’s never seen before — it begins making antibodies against it. For this reason, it may later mistake a fetus who’s Rh positive for an unwelcome invader and develop an immune reaction that actually attacks it.

If you’re Rh negative and your partner is Rh positive, there’s a chance that your child will be positive, making incompatibility a concern. If you’re Rh positive, it really doesn’t matter if your child is negative or positive. If you’re Rh positive, your body is already familiar with the Rh protein, and so the baby’s Rh status will not be an issue. And if both you and your partner are negative, you don’t have to worry about Rh incompatibility because your child will be negative, too.

Rh incompatibility isn’t usually a problem during your first pregnancy because blood from a developing fetus doesn’t mingle with your own until the time of delivery. The immune system of an Rh negative mom has to be exposed to Rh positive blood one or more times before it can develop a response that is strong enough to harm the baby. But during labor and delivery, you will probably be exposed to some of your child’s blood. If you’re Rh incompatible, your body will start making antibodies against this “foreign” protein.

These antibodies themselves are generally harmless until you have a subsequent pregnancy. However, there are some situations in which a problem could arise even in what seems like a first pregnancy — that is, if you’ve had a miscarriage, abortion, or an invasive procedure such as an amniocentesis or chorionic villus sampling. That’s why doctors recommend preventive treatment against Rh incompatibility in these cases.

If you become pregnant with a second Rh-positive baby, your antibodies will set out to do their job — with potentially devastating results. Antibodies can cross over into the baby’s bloodstream and wipe out any cells they identify as foreign. This may cause your baby’s red blood cells to swell and rupture — a condition called Rh disease. The illness can cause jaundice (yellowing of the skin and eyes), and anemia (low levels of red blood cells). In some cases, the disease can lead to brain damage, heart failure, or even death. Fortunately, modern medicine has dramatically reduced the risks of Rh disease.

According to the March of Dimes, fetuses with Rh disease can be treated in the uterus at 18 weeks with blood transfusions, and about 90 percent of treated babies now survive.

How can I protect my baby from Rh disease?

In the past, Rh incompatibility was a serious problem; newborns diagnosed with Rh disease often became seriously ill or died. Since the 1960s, scientific breakthroughs have made it possible to prevent Rh disease and treat Rh incompatibility.

A Doppler ultrasound is more accurate than amniocentesis in detecting the severe anemia associated with Rh disease, according to a 2006 study. If your fetus shows signs of Rh disease, he or she can receive blood transfusions while still in the womb. Your doctor will talk with you about other treatment options, including early delivery. An Rh-positive infant will most likely need special care at birth.

The best way to treat Rh disease is to prevent it completely. If you’re Rh negative and you’re carrying an Rh positive baby, your doctor can lower the risk of future trouble by giving you a shot of a blood product called Rh immune globulin (RhIg) within 72 hours of delivery. This injection will clear any Rh proteins that reach your body before your immune system has a chance to mount an attack. This treatment is effective in more than 95 percent of Rh-negative women.

As an extra precaution, your doctor will probably recommend an RhIg injection around the 28th week of pregnancy. (This can prevent rare cases where a woman starts producing Rh antibodies months before delivery.) This drug is considered safe for pregnant women. The only known side effects are soreness at the injection site or a slight fever. There is no risk of HIV transmission through this purified blood product.

If you are Rh negative and you experience miscarriage, ectopic pregnancy, bleeding during pregnancy, or have an abortion, you should also receive RhIg. This treatment is also recommended after certain prenatal tests, including amniocentesis and chorionic villus sampling (CVS). Repeat injections are necessary with future pregnancies, as the medication appears to offer protection for only about three months.

These shots may not be much fun, but for Rh negative women who want to avoid Rh disease in their newborns, it may be small price to pay for peace of mind.


American College of Obstetricians and Gynecologists. Planning Your Pregnancy and Birth. Third Edition.

American College of Obstetricians and Gynecologists. Routine Tests in Pregnancy. ACOG Patient Education Pamphlet.

Mayo Clinic. Rh-negative mothers: Can blood type be harmful to the fetus? Last updated April 2010.

American Pregnancy Association. Rh-factor. http://www.americanpregnancy.org/prenataltesting/rhfactor.html

American Academy of Family Physicians. What is Rh Incompatiblity? http://www.kidshealth.org/

March of Dimes Foundation. Rh Disease.

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