Rh Factor in Pregnancy

Discovering Your Blood Type and Rh Factor

You've discovered you are pregnant and now you have some routine blood tests that are done to determine your blood type and your Rh status. Rh (Rhesus) factor refers to a specific antigen in the blood found on the surface of the red blood cells. If you have the protein, then you are Rh-positive, as are most people in the world. If you do not have the antigen, then you are Rh-negative and that means you will have to take some precautions during your pregnancy.

When a mother is Rh-negative and the father is Rh-positive, the baby can inherit the Rh factor from the father, making the baby Rh-positive as well. If you are Rh-negative there's a pretty good change your blood is incompatible with your baby's blood, which will likely be Rh-positive. There is no way to know the baby's Rh factor until birth, but it's safe to assume it is Rh-positive. The statistics indicate that if the father is Rh-positive and the mother Rh-negative, there's a 70% chance of having an Rh-positive baby. If both parents are Rh-negative, then the baby will be Rh-negative as well.

What Happens When Blood Mingles

If this is your first baby, then being Rh-incompatible probably will do no harm to you or your baby. However, if your baby's blood leaks into yours, as can happen at certain times during pregnancy or birth, your immune system will kick in and produce antibodies against the Rh-positive blood. Should that occur, you become Rh-sensitized and the next time you become pregnant you body will read the presence of the baby as a foreign intruder and it may attack your baby's blood. The great news is that you can neutralize this with an injection of a drug called Rh immune globulin at any point there is danger of your blood being exposed to your baby's blood.

Taking Precautions

Normally in pregnancy the baby's blood stays separate from the mother's blood and very few cells cross the placenta. Actually, it is unlikely the blood will mingle until the time of delivery. That's why Rh-negative factor isn't usually a problem with the first pregnancy. If your blood mixes during delivery you'll have given birth before your body has a chance to make antibodies against the Rh-positive blood, so there will be no problems.

A shot will be necessary is your baby is Rh-positive (which will be discovered after the baby is born). If you were exposed to Rh-positive blood during delivery, the shot will prevent your body from making any antibodies that could attack a future baby with Rh-positive blood. The blood samples taken from the baby's heel and the cord are tested for several things, among them Rh factor. Without treatment there's a 15% chance you'll develop antibodies, but with the shot, the risk is virtually nil.

When It Is Too Late for the Shot

If you are Rh-negative, have been pregnant before but didn't the shot, a routine prenatal blood test will tell whether you have the antibodies that attack Rh-positive blood. If so, then it is too late to get the shot and, if the baby is Rh-positive, then some problems may be on the horizon for him. If you don't have the antibodies, then the shot will protect you from developing them. There are a few situations in which your baby's blood might mix with yours. If you miscarry or have an abortion, or an ectopic or molar pregnancy the risk is there. An invasive procedure like amniocentesis or chorionic villus sampling increases the chances as well. Vaginal bleeding, injury to the abdomen during pregnancy and stillbirth are other situations in which the baby's blood may mix with yours.

Proper treatment can ensure a safe and healthy delivery for both mother and baby.


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