Rh disease was once a significant problem in obstetrics, affecting upward of 1% of pregnancies. It is still a pregnancy complication but, fortunately, a rare one. With the development of Rhogam in 1968, an immunoglobulin that prevents the body from making antibodies that trigger Rh disease, the number of cases has dropped tenfold.

We need to have a basic understanding of a patient's blood type and Rh status in order to understand this disease. As you know, our blood type is either A, B, AB or O. We also have another factor called the rhesus factor (named after the rhesus monkey), which is either positive or negative. The combination gives each of us our blood type. About 85 percent of Caucasian Americans and 93 percent of African Americans are Rh positive. The rest are Rh negative.

Below you'll see how the rhesus factor in a woman's blood affects the possibility of her developing Rh disease.

• An Rh-positive mom will not have Rh disease, regardless of her partner's Rh status.
• An Rh-negative mom and an Rh-negative partner will not have result in Rh disease.
• An Rh-negative mom and an Rh-positive partner can result in Rh disease.

(Your partner can find out his blood type by getting a blood test or by donating blood.)

How does Rh disease occur? During pregnancy, a few of baby's blood cells manage to escape into the mother's circulation. This happens in almost every pregnancy. However, Rh-positive baby cells are perceived by the mom's system as foreign, just like a virus or bacteria. In response, she makes antibodies that attack these cells. The antibodies can cross the placenta and attack the baby's red blood cells. However, with the first pregnancy, the mother doesn't make enough antibodies to cause serious problems, such as fetal anemia.

In subsequent pregnancies, the baby can be severely affected. In these cases, doctors can carefully monitor the baby with ultrasound, use amniocentesis in the second and third trimester to check on the baby's status or take blood from the baby's umbilical cord to test for anemia. In some cases, blood transfusions given to the baby while in the uterus can save his or her life. Specialized centers are well-equipped to handle these rare cases.

Giving Rhogam at prescribed intervals during pregnancy to those at risk has reduced the cases of Rh disease across the United States. Rhogam actually sweeps up any errant baby cells that might slip into the mother's blood. This reduces the likelihood that mom will produce Rh antibodies.

Prevention is the key to reducing this obstetrical nightmare. The obstetrician will manage an Rh-negative woman as follows:

1) Check the mom's initial lab tests to see if she is already sensitive to Rh-positive blood.

2) Test the partner's blood type and Rh status.

3) Retest the mom to make sure she is not sensitized to Rh-positive blood before giving a dose of Rhogam. Rhogam is given at an amniocentesis, at week 28 of pregnancy and, if she has not delivered, at 41 weeks of pregnancy. If the mom experiences vaginal bleeding early in her pregnancy or has a miscarriage, she should discuss the possibility of receiving Rhogam with her practitioner.

4) Check the baby's Rh status at delivery, and give Rhogam if the baby is Rh-positive.

If you have any questions about your Rh status or risk of having Rh disease, talk to your practitioner.