If you are at risk for having a baby with a genetic defect or blood disorder, your doctor may recommend percutaneous umbilical cord blood sampling (PUBS), which is performed at 18 weeks' gestation.

During the procedure, the doctor draws a sample of the fetus' blood directly from the umbilical cord. The sample is then analyzed for genetic defects and other health conditions.

This procedure can also be used to check if your baby has anemia. If your baby has severe anemia, your health care provider can immediately give your baby a blood transfusion.

Guided by ultrasound, your doctor pinpoints the spot where the umbilical cord meets the placenta. He then inserts a needle through your abdomen and uterus and into the umbilical cord and withdraws a small amount of fetal blood.

There are two routes for retrieving fetal blood: Placing the needle through the placenta or through the amniotic sac. The placenta's position in the uterus and the spot where it connects to the umbilical cord determine which method your doctor uses.

If the placenta is attached toward the front of the uterus (placenta anterior), he inserts the needle directly into the umbilical cord without passing through the amniotic sac. The amniotic sac, or "bag of waters," is the fluid-filled structure that cushions and protects the developing fetus.

If the placenta is attached toward the back of the uterus (placenta posterior), the needle must pass through the amniotic sac to reach the umbilical cord. This may cause some temporary bleeding and cramping.

You should receive Rh immune globulin (RHIG) at the time of PUBS if you are an Rh-negative unsensitized patient.

After the blood sample is drawn, it's sent to a laboratory where a technician tests the blood for genetic defects and other disorders. In most cases, the results are ready within a few weeks. Your doctor will explain the results to you and, if a problem is diagnosed, give you information about ending the pregnancy or how best to care for your baby after birth.