About your Amniocentesis

Download the consent for amniocentesis

What is amniocentesis?
Amniocentesis is an invasive prenatal test that allows your health care practitioner to gather genetic information about your baby by obtaining a sample of your amniotic fluid. This is the fluid that surrounds your baby in the uterus.

The most common reason to have an “amniocentesis” is to determine whether a baby has a genetic disorder or a chromosomal abnormality, such as Down syndrome. Only amniocentesis or chorionic villus sampling (CVS) can diagnose these problems in the womb.

Amniocentesis is usually done when a woman is between 16 and 20 weeks pregnant. Women who choose to have this test are primarily those at increased risk for genetic and chromosomal problems, in part because the test is invasive and carries a small risk of miscarriage.

Here are a few other reasons that amniocentesis may be done:

• To determine whether your baby’s lungs are mature enough for an early delivery if you appear to be in premature labor or require an early delivery for any reason.

• To diagnose or rule out a uterine infection if, for instance, your water has broken prematurely.

• To check on the well-being of your baby if you have a blood sensitization, such as Rh sensitization. This is a complex condition that can occur if your blood is a different type than your baby’s. (Note: Obstetricians are increasingly using Doppler ultrasound for this purpose instead of amniocentesis.)

What’s amniocentesis like?

Before you have your amniocentesis , you’ll have an ultrasound to measure your baby and check his/her basic anatomy.

For the amniocentesis itself, you’ll lie on an examining table and your belly will be cleaned with alcohol or an iodine solution. Ultrasound is used to pinpoint a pocket of amniotic fluid a safe distance from both the baby and the placenta.

Then, under continuous ultrasound guidance, our doctor will insert a long, thin, hollow needle through your abdominal wall and into the sac of fluid around your baby. A small amount of amniotic fluid will then be withdrawn — about two tablespoons — and then remove the needle.

Withdrawing the fluid can take a few minutes but usually takes less than 30 seconds. Your baby will make more fluid to replace what’s taken out.

You may feel some cramping, pinching, or pressure during the procedure — or you may feel little discomfort at all. The amount of discomfort or pain varies among women and even from one pregnancy to the next.  Local anesthetic is generally not used because the discomfort with the anesthetic is usually more uncomfortable than the amniocentesis alone.

Because ultrasound guidance is used during the procedure, direct injury to the baby from amniocentesis is very rare. Our doctor will avoid placing the needle near the baby.

Afterward, the baby’s heartbeat will be evaluated.

Note: If your blood is Rh-negative, you’ll need a shot of Rh immunoglobulin after amniocentesis (unless the baby’s father is Rh-negative as well) because your baby’s blood may have mixed with yours during the procedure and it may not be compatible.

What is the risk of miscarriage from amniocentesis?
Although it’s often estimated to be between one in 300 and one in 500, there’s no real consensus on the actual risk of miscarriage due to amniocentesis. One large recent study estimated the risk to be as low as one in 1,666, although many experts dispute the methodology used to arrive at this figure.  This figure is very different from all other studies which have used pooled data from experienced centers.

Because a small percentage of women will end up miscarrying in the second trimester anyway, there’s no way of knowing for sure whether a miscarriage following an amniocentesis was actually caused by the procedure.

What happens after the procedure?
You’ll need to take it easy for the rest of the day.  Avoid any heavy lifting, intercourse, and air travel for the next 24 hours.

You may have some minor cramping for an hour or so. If you have significant cramping or vaginal spotting, or you’re leaking amniotic fluid, call your practitioner immediately.  Also call right away if you have a fever, which could be a sign of an infection.

When will I get the results?

Most of the time you can get preliminary results while you’re waiting for the cells to reproduce. For example, in most cases, a technique called fluorescence in situ hybridization (FISH) may be used to look for select problems. Preliminary results are available quickly, often in a couple of days.

You should have the full results within two weeks. During this period, a laboratory will culture or grow the cells collected in the amniotic fluid and then the lab will analyze the cells.  This type of analysis will determine if your baby has a more rare and unusual chromosomal abnormality.

What happens if my baby is found to have a problem?
You will be offered genetic counseling so that you can get more information and discuss your options. Some women opt to interrupt the pregnancy, while others decide to continue. Whichever route you choose, you may find that you want further counseling or support. Some women find support groups helpful, others may want individual counseling, and some may choose both. Be sure to let your nurse midwife, obstetrician, perinatologist, nurse, or sonographer know if you need more help so they can give you the appropriate referrals.