Amniocentesis is a test that is performed by inserting a thin, hollow needle into the amniotic fluid and then withdrawing some of the fluid through the needle into a syringe. The amniotic fluid can then be tested in a variety of ways.
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If your practitioner performs a genetic amniocentesis — a check of fetal chromosomes — he usually conducts it at 15 to 20 weeks. Your practitioner may perform amniocentesis for other reasons, like checking for lung maturity in the baby, at any time later in the pregnancy.
During the amniocentesis procedure, you lie flat on your back on top of a table. Your doctor cleans your abdomen with an iodine solution. Using ultrasound to locate an area of amniotic fluid that is away from the baby, your doctor inserts a thin needle through your abdomen and uterus into the amniotic sac. After withdrawing enough amniotic fluid (usually about 15 to 20 cc, or 1 to 2 tablespoons), he removes the needle.
A common misconception is that the needle is inserted through the navel. The exact point of insertion depends on where the fetus, the placenta, and the amniotic sac are located within the uterus. You may have heard that the amniocentesis needle is exceptionally long, and you may be afraid of long needles.
But the needle’s length, which enables it to reach the amniotic sac, doesn’t make it painful. A needle’s thickness determines how uncomfortable it is, and an amniocentesis needle is very thin.
Having an amniocentesis performed isn’t altogether pain-free, but most women report that it isn’t as bad as they expected it to be. Afterward, your doctor may advise you to rest and avoid strenuous activity and sex for one to two days. Most women cramp a bit the day of the procedure, which is expected and normal.
A genetic amniocentesis primarily tests to see that 23 chromosome pairs are present and that their structure is normal. It doesn’t routinely test for all possible genetic diseases or birth defects. The amniotic fluid cells must be incubated before your doctor can read the results of a genetic amniocentesis. Results are usually available in one to two weeks.
If prenatal blood studies show that you’re Rh-negative, your doctor will give you an injection of Rh-D immune globulin (Rhogam and Rhophylac are two examples currently on the market), which helps prevent Rh sensitization.
Risks and side effects of amniocentesis
Not all patients have these symptoms or problems after an amniocentesis, but remember that they can occur:
Cramping: Some women experience cramping for several hours after the procedure. The best treatment for this cramping is rest. Some practitioners recommend a single glass of wine to help ease the discomfort.
Spotting: This may last one to two days.
Amniotic fluid leak: A leakage of 1 to 2 teaspoons of fluid through the vagina occurs in 1 to 2 percent of patients. In the great majority of these cases, the membrane seals over within 48 hours. Leakage stops and the pregnancy continues normally. If you experience a large amount of leakage or persistent leakage, call your doctor.
Fetal injury: Injury to the fetus is extremely rare, given the use of ultrasound guidance.
Miscarriage: Although amniocentesis is considered very safe, it’s still invasive and is associated with a small risk of pregnancy loss. Recent studies have shown that the risk of pregnancy loss after amniocentesis is much less than the 0.5 percent risk previously quoted and is probably closer to about 1 in 1,000.
Your decision to undergo the procedure must weigh both risks and benefits, which vary according to the individual.
An amniocentesis performed later in the pregnancy — later than 20 weeks — doesn’t carry the same increased risk of miscarriage. It carries only a very small risk of infection, rupture of membranes (breaking the water), or onset of labor.
Reasons for having an amniocentesis
Your practitioner may recommend a genetic amniocentesis for the following conditions or situations:
Your age is 35 or more at your due date (even though the recommendations have changed, some practitioners still recommend amniocentesis for all women over the age of 35). This age recommendation also depends on the number of babies you’re carrying. For example, if you’re carrying twins, your practitioner may offer an amniocentesis at age 33.
You had an elevated MSAFP.
You had abnormal results from Down syndrome screening (during either your first or second trimester).
Your ultrasound exam was abnormal, indicating, for example, poor fetal growth or suspected structural abnormalities.
You had a previous child or previous pregnancy with a chromosomal abnormality.
You’re at risk of having a baby with a certain genetic disease.
You and your partner have concerns and want to confirm that the chromosomes are normal.
Your practitioner may perform amniocentesis for other reasons:
Preterm labor: An infection within the amniotic fluid may be a cause of preterm labor. Your practitioner can send the fluid to a lab for tests to look for any such infection.
Other infections: Some patients may find that they’re at risk of developing infections such as toxoplasmosis, CMV (cytomegalovirus), or parvovirus. The amniotic fluid can be tested for evidence of such problems in patients at risk.
Rh sensitization: Patients with Rh sensitization are sometimes monitored with a test known as delta OD-450, in which the amniotic fluid is examined for evidence of broken-down fetal red blood cells.
Lung maturity studies: Sometimes your doctor needs to find out whether the fetus’s lungs are mature enough for the baby to be delivered. Certain tests on the amniotic fluid can determine the maturity of the lungs.