Cordocentesis — also known as percutaneous umbilical blood sampling — is a diagnostic prenatal test in which a sample of the baby's blood is removed from the umbilical cord for testing.

Cordocentesis, which is usually done after week 18 of pregnancy, can be used to detect certain genetic disorders, blood conditions and infections. Cordocentesis can also be used to deliver blood and medication to a baby through the umbilical cord.

Use of cordocentesis is becoming rare because diagnostic procedures such as amniocentesis and chorionic villus sampling, which pose a lower risk of fetal death, can be used instead for prenatal diagnosis of disease. Cordocentesis is most often done to test for anemia in the baby.

Cordocentesis is used primarily to detect and treat blood conditions, such as fetal anemia — a low amount of healthy red blood cells in a developing baby.

Cordocentesis is usually done when a diagnosis can't be made from amniocentesis, chorionic villus sampling, ultrasound or other methods. Cordocentesis carries a higher risk of complications to the baby, including death, than other procedures do. Your health care provider will offer the procedure only if other options aren't available or they won't produce results quickly enough.

Rarely, cordocentesis might be used to check fetal chromosomes through chromosome microarray or karyotype analysis. Blood obtained through cordocentesis can also potentially be used for other types of genetic studies.

Cordocentesis carries potentially serious risks, including:

• Fetal bleeding. Bleeding from the area where the needle is inserted is the most common complication. If life-threatening fetal bleeding occurs, your health care provider might recommend replacement of blood products to the fetus.

• Cord hematoma. A collection of fetal blood within the cord might occur during or after a cordocentesis. Most babies don't have signs or symptoms when this occurs. However, a few might develop a low heart rate for a short period.

  If the hematoma is stable, your health care provider will observe the baby. If the hematoma isn't stable or if your baby's heart rate doesn't recover, your health care provider will recommend an emergency cesarean delivery.

• Slowing of the baby's heart rate. The baby's heart rate might slow temporarily after cordocentesis.
• Infection. Rarely, cordocentesis can lead to a uterine or fetal infection.
• Fetal-maternal bleeding. Fetal blood might enter maternal circulation in about 40 percent of procedures. The amount of bleeding is usually small. This problem is more common when the placenta lies in the front of the uterus.
• Passing maternal infection. If the mother has certain infections, such as hepatitis B, hepatitis C or HIV, they might be passed to the baby.

• Pregnancy loss. Cordocentesis carries a higher risk of fetal death than do other prenatal diagnostic tests, such as chorionic villus sampling and amniocentesis. The risk is about 1 to 2 percent for a fetus that appears normal and is being tested for genetic disorders.

  However, since many babies are ill when the test is done, it's often difficult to determine whether fetal death is related to the procedure or to the baby's health.

Ultimately, the decision to have cordocentesis is up to you. Your health care provider and a genetic professional can help you weigh the risks and benefits.

If you are 23 or more weeks pregnant, you'll be asked to avoid eating or drinking after midnight the night before cordocentesis. This is because certain complications caused by the procedure might require an emergency cesarean section.

You might want to ask your partner or a friend to accompany you to the appointment for emotional support or to drive you home afterward.

Before week 23 of pregnancy, cordocentesis is usually done in an outpatient facility or the health care provider's office. After week 23 of pregnancy, cordocentesis is usually done in the hospital, in case the baby develops complications that might require an emergency delivery.

A sample of your blood will be taken before the procedure for comparison with the fetal blood samples.

During the procedure

About 30 to 60 minutes before the procedure, you might be given antibiotics to reduce the risk of a uterine infection. This is usually done through a tube inserted into a vein.

Your health care provider will use ultrasound to determine the umbilical cord's location in your uterus. You'll lie on your back on an exam table, and your health care provider will apply a special gel to your belly. He or she will then use a small device known as an ultrasound transducer to show your baby's position on a monitor.

Next, your health care provider will clean your belly. Sometimes medication is given to prevent discomfort during the procedure, but often it isn't needed.

Guided by ultrasound, your health care provider will insert a thin, hollow needle through your abdominal wall and into your uterus. A small amount of blood from the vein in the umbilical cord will be withdrawn into a syringe, and the needle will be removed.

You'll need to lie still while the needle is inserted and the blood is withdrawn. You might notice a stinging sensation when the needle enters your skin, and you might feel cramping when the needle enters your uterus.

After the procedure

After the blood sample is taken, you might have cramping or a small amount of discomfort.

Your health care provider will use an ultrasound or an external labor monitor to track your baby's heart rate after the procedure.

When you go home, your health care provider might suggest resting for the remainder of the day. You'll likely be able to resume normal activities the next day. Call your health care provider if you experience vaginal bleeding or fluid leakage.

The blood sample will be analyzed in a lab. Test results are typically available within days.

Your health care provider or a genetic professional will help you understand the results of your cordocentesis. If your test results are normal, your health care provider will discuss the need for any follow-up appointments.

If your baby has an infection, your health care provider will explain the treatment options. If your baby has severe anemia, he or she might need a blood transfusion through the umbilical cord.

If your test results indicate that your baby has a condition that can't be treated, you might be faced with wrenching decisions — such as whether to continue the pregnancy. Seek support from your health care team, your loved ones and other close contacts during this difficult time.