Amniocentesis, Amniotic fluid bilirubin optical density, OD
What is this test?
This test is done to see how well your baby is developing inside you. The liquid that surrounds the baby inside your womb is called amniotic fluid. Health care providers check the fluid for a pigment called bilirubin to see whether your baby has a blood type that is incompatible with yours. When this happens, it may cause the baby to develop severe anemia or other problems.
If your baby has red blood cell proteins that you do not have, your body may later mistake the baby for an intruder and attack it through your immune system. This only happens to mothers who are Rh-negative and carry babies who are Rh-positive. This is why experts recommend that you get tested on your first prenatal visit, or even earlier, to find out if you are Rh-negative. If you find out that your and your baby's blood types are not compatible, you can get treatment that will protect your baby.
You may already know whether you have blood type A, B, O, or AB. If you have had a blood type test, you might also know if you are Rh-positive or Rh-negative. This tells you whether you have a certain type of protein – or antigen – called Rh factor on the surface of your red blood cells. If your red blood cells don't have this protein, you are Rh-negative. If they do, you are Rh-positive.
Your doctor will test your blood to see whether you are Rh-negative. If your baby is Rh-positive or has other red cell proteins that you do not have, your body may make antibodies against your baby's blood. Your antibodies can cause some of your baby's red blood cells to break down. This can lead to serious blood problems in your developing baby. When this happens, the bilirubin levels in the fluid surrounding the baby go up. Testing the fluid for bilirubin levels will show whether the baby has lost too many red blood cells. This way, health care providers can start treatment to help your baby.
Your health care providers may suggest this test if:
- You are Rh-negative
- You were exposed to red blood cell proteins you do not have through an earlier delivery, miscarriage, or blood transfusion
- Blood tests show that you have increasing levels of antibodies to Rh factor in your blood
Testing for bilirubin may be done after 25 weeks of pregnancy. If the lab finds evidence that your baby has an abnormal breakdown of red blood cells, health care providers may repeat this test as often as every 14 days.
What other tests might I have along with this test?
You may have a blood test to measure your Rh antibody levels. Before doing the amniocentesis, health care providers may do an ultrasound to check the location of your baby. Ultrasound is a type of imaging study that uses sound waves. After 30 weeks, a doctor may also check your amniotic fluid to find out how mature your baby's lungs are. This test is called an L/S ratio.
What do my test results mean?
Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.
Bilirubin is a pigment, so it can change the color of amniotic fluid, making it amber or yellow. The most common way bilirubin is measured in amniotic fluid is by determining its optical density (OD).
- A level of 0.28 OD to 0.46 OD at 28 to 31 weeks is considered low. This means your baby likely does not have an abnormal red blood cell breakdown or may have only a very mild anemia.
- A level of 0.47 OD to 0.90 OD means your baby may have moderate red cell disease. If so, the baby will likely need a blood transfusion in the womb. Your health care provider may also recommend an early delivery if your pregnancy has reached 32 or 33 weeks.
- A level of 0.91 OD to 1.0 OD means your baby is at risk for severe anemia from the breakdown of red blood cells. Health care providers call this "hemolytic anemia.". Your baby may need a blood transfusion in the womb. Your health care provider may also recommend an early delivery at 32 to 33 weeks.
- A level of less than 0.04 OD toward the end of your pregnancy means your baby is reaching maturity and is not in danger of this type of anemia.
How is this test done?
This test removes a tiny amount of the fluid in your womb for testing. This is known as amniocentesis. The test may be done at the hospital or at your doctor's office. A health care provider will first rub or inject a numbing medicine into your belly. He or she will then place a long, very thin needle into your womb and take out about 5 to 10 milliliters of fluid (about 1 to 2 teaspoons). Your body will replace this fluid in a few hours. After amniocentesis, your health care provider may ask you not to have sex or exercise for the rest of the day. Otherwise you may return to all your normal activities.
Does this test pose any risks?
Amniocentesis is a safe procedure. But it carries a small risk of damage to the baby, miscarriage, bleeding, infection, or leaking of amniotic fluid. You should discuss these risks with your health care provider. You may feel a stinging sensation when the needle goes in. You may also feel some pressure or discomfort as the fluid is withdrawn. Afterward, you may have some mild cramping. Having amniocentesis may increase your risk of developing antibodies during a future pregnancy. To reduce this risk, your health care provider may give you an injection of a medication called Rho (D) immune globulin (RhoGAM).
What might affect my test results?
Hemolytic anemia from the breakdown of red blood cells is the most common cause of abnormal test results. Other types of fetal problems or diseases that destroy red blood cells in the womb may lead to abnormal results too. If you have hepatitis or a sickle cell crisis while pregnant, this may also influence the results.
Your test results may be affected if you have taken any steroid medications. Other factors that can affect the results include:
- Blood in the fluid sample
- Stool from the baby in the fluid sample
- Exposure of the fluid sample to light
- Fetal acidosis, which means the baby's blood plasma is too acidic
How do I get ready for this test?
If you are having an ultrasound before the amniocentesis, you should drink fluids 1 hour before and arrive for the test with a full bladder if your pregnancy is 20 weeks or less. If you are 21 weeks or beyond you do not need to drink extra fluids and do urinate before the test. No other preparations are needed.