Feature Archive

He's Positive, She's Negative: What's That Do to Baby?

He's Positive, She's negative; What's That Do To Baby?

By Carolyn Strange
WebMD Feature

A: It's always a good idea for any couple to think ahead and prepare for pregnancy, so Mom and baby can be as healthy as possible. When facing the potential for Rh disease, as you two are, it's even more important. You'll probably want to educate yourselves about Rh incompatibility. And in any case, make sure you find a health-care provider who understands Rh disease, and with whom it's easy to communicate.

One thing is clear -- your baby will have type O blood. What's not clear is whether your baby's blood will be Rh-positive or Rh-negative, and that's what makes all the difference.

Rh disease of the newborn arises from incompatibility of the Rh factor between the mother and baby. It's a bit simplistic, but you can think of the Rh factor as a protein that is either present (positive) or absent (negative) on red blood cells. Exact percentages vary with race, but most people are Rh-positive.

A woman with Rh-negative blood has nothing to worry about if her baby is also Rh-negative, and a woman with Rh-positive blood need not worry at all. Problems arise only with Rh-negative mothers and Rh-positive babies. Usually the first pregnancy goes fine. It's a subsequent Rh-positive baby who may be at risk. The mother herself is in no danger.

Normally, maternal and fetal blood supplies don't mix during pregnancy, but during childbirth, some fetal blood may enter the mother's system. If the mother is Rh-negative and the fetus is Rh-positive, the woman's immune system responds with antibodies to the Rh factor. The chances of responding, and the strength of the response, increase with each Rh-positive pregnancy. In a subsequent pregnancy these antibodies cross the placenta and enter fetal circulation. If the next fetus is also Rh-positive, the mother's antibodies destroy fetal red blood cells. The baby may be born anemic or jaundiced, and in severe cases many fetuses have died.

Although treatments are available to save affected babies - including transfusing Rh-negative blood, sometimes even prior to birth - prevention obviously makes more sense. The trick is to block the mother's immune system from becoming sensitized to the Rh factor.

An injection of anti-Rh antibodies (widely known by the trade name RhoGAM) given to the mother soon after birth neutralizes any fetal blood cells in her circulation before her immune system has a chance to respond. Subsequent pregnancies should be like the first, as if the woman was never exposed to the Rh factor. That's the theory, and quite often things work just that smoothly.

Now for some real-life complexities. RhoGAM is useless if a woman is already sensitized. Any pregnancy event with the potential for fetal-maternal blood mixing can sensitize the mother. That includes certain placental abnormalities, tubal pregnancy, miscarriage and invasive procedures such as abortion or amniocentesis. The chances of mixing and sensitization are lower earlier in pregnancy, but there's still a risk.

Most experts recommend a RhoGAM shot at 28 weeks to head-off sensitization, as well as after birth. RhoGAM doesn't hurt the fetus because there are different kinds of antibodies and the ones in RhoGAM are a type that won't cross the placenta, so never reach the fetus. Once a woman has had this shot, she should make sure everyone involved in her health care knows. Otherwise, when she has blood tests, they might wrongly assume that she has become sensitized.

RhoGAM shots aren't necessary if the fetus has Rh-negative blood, but that usually isn't known until birth. An amniocentesis at 18 weeks can tell you, but also carries a small risk of sensitization. "When they do an amnio, the doctor should know she's Rh-negative and try not to go through the placenta," says Dr. Amos Grunebaum, director of Maternal-Fetal Medicine at St. Luke's-Roosevelt Hospital Center in New York, and a vice president of OnHealth.com. "They should go to a doctor who will only stick once, and with the smallest possible needle," he says.

In your case, whether your baby has Rh-negative or Rh-positive blood depends on your genes. You can be Rh-positive two ways. You might be what's called homozygous, meaning you carry two positive Rh-factor genes, one from each of your parents. If so, your baby will have Rh-positive blood. Or you might be what's called heterozygous, meaning you carry one negative and one positive gene. In that case, your baby has a 50/50 chance of having Rh-positive blood.

If you happen to know that one of your parents is Rh-negative, then you know you have one negative gene and that you're heterozygous. If both your parents are Rh-positive, you can't assume anything, because, like you, they might be either heterozygous or homozygous, and you have no way of knowing which genes you got.

Some people worry that RhoGAM is a blood product. "Nobody has ever gotten AIDS or hepatitis from it," Dr. Grunebaum says. You may hear that sometimes even with RhoGAM a woman becomes sensitized. That can happen, and it's unfortunate, but it's no reason to avoid the shot. Or you may hear that some Rh-negative women have given birth to multiple Rh-positive babies, without benefit of RhoGAM, and everyone was fine. That can happen, too, but it's no reason to take chances. The benefits of RhoGAM seem to far outweigh the risks, but you'll want to discuss this when you find that knowledgeable, communicative doctor.

©1996-2005 WebMD Inc. All rights reserved.
Last Editorial Review: 1/30/2005 11:11:46 PM