Pregnancy and Amniocentesis

Medically Reviewed by Zilpah Sheikh, MD on February 12, 2024
6 min read

During pregnancy, the fetus is surrounded by amniotic fluid, a substance much like water. Amniotic fluid has live fetal cells and other things, such as alpha-fetoprotein (AFP). They provide important information about your baby's health before birth.

Your doctor may suggest a test called amniocentesis (or amnio) if your baby has a higher chance of some birth defects. You may also need it if you have signs of an infection or if you may deliver early. Unlike blood testing, which shows only whether you are at risk, doctors use amniocentesis to make a diagnosis.


Amniocentesis is an invasive prenatal test in which your doctor takes a sample of the amniotic fluid. They remove the sample (less than an ounce) through a fine needle inserted into the uterus through your belly, under ultrasound guidance. A lab tests the sample, checking your baby’s chromosomes. Those tests can include the karyotype test, the FISH test, and microarray analysis.

You'll have a complete anatomical ultrasound before an amniocentesis, but you may also need amniocentesis to look for certain birth defects, such as Down syndrome, a condition where your baby has an extra chromosome.

Amniocentesis carries a small risk for both you and your baby, so doctors usually offer it to people with a higher chance of genetic diseases, including those who:

  • Have an abnormal ultrasound or abnormal lab screens
  • Have a family history of certain birth defects
  • Have previously had a child or pregnancy with a birth defect
  • Had an abnormal genetic test result in the current pregnancy 

Amniocentesis doesn't find all birth defects, but doctors can use it to spot these conditions where the parents have a significant genetic risk:

Amniocentesis can discover certain neural tube defects (diseases where the brain and spinal column don't develop properly), such as spina bifida and anencephaly

Because you'll have an ultrasound at the time of your amniocentesis, it may spot birth defects undetected by amniocentesis (such as cleft palate, cleft lip, club foot, or heart defects). But there are some birth defects that amniocentesis or ultrasound won't find.

If you're having an amniocentesis, you may ask to find out the baby's sex; amniocentesis is the most accurate way.

Although you'll rarely have it in the third trimester, an amniocentesis can figure out if your baby's lungs are mature enough for delivery and check the amniotic fluid for infection.

When do you get an amniocentesis?

If your doctor has suggested an amniocentesis, you'll usually have it between your 15th and 18th weeks of pregnancy.

How accurate is amniocentesis?

Amniocentesis is about 99.4% accurate. It may sometimes be unsuccessful because of technical problems like trouble collecting enough amniotic fluid or growing the collected cells when cultured.

Amniocentesis vs NIPT

Noninvasive prenatal testing (NIPT) helps to figure out your baby's chances of being born with certain disorders affecting their chromosomes. Unlike amniocentesis, NIPT doesn't diagnose conditions. The purpose of the test is to tell you whether there's a chance of your baby having a genetic condition. NIPT is also noninvasive; you'll only need a blood test.

There is a small chance that an amniocentesis could cause a miscarriage (less than 1%, or approximately 1 in 1,000 to 1 in 43,000). Injury to you or your baby, infection, and preterm labor are other possible problems that can happen but are extremely rare. Your chances of problems are higher if you're carrying twins. Other potential health problems with amniocentesis include:

  • A less than 1 in 1,000 chance of severe infection
  • Rhesus disease, a condition where antibodies in your blood wipe out your baby's blood cells.
  • A higher chance of your baby forming a club foot

Do I have to have an amniocentesis?

No. You will receive genetic counseling before the procedure. After the counselor has fully explained the pros and cons of amniocentesis, you can choose whether or not to have it.

Instead of an amniocentesis, you can choose to have a procedure called chorionic villus sampling (CVS). The chorionic villi are the building blocks of the placenta and have the same genetic makeup as your developing baby. 

Amniocentesis vs. CVS

Like an amnio, CVS diagnoses birth defects before your baby is born. One benefit of CVS is that you can do the procedure in your first trimester, at 10 to 13 weeks of pregnancy.

Health care staff will clean a small area of your belly with an antiseptic to get ready for the amniocentesis. You may get a local anesthetic (pain-relieving medication) to ease any discomfort. The doctor first finds the position of the fetus and placenta with an ultrasound. Under ultrasound guidance, the doctor inserts a thin, hollow needle through your belly and uterus, and into the amniotic sac, away from the baby. Then they'll remove a small amount of fluid (less than an ounce) through the needle and send it to a lab for analysis.

If you’re carrying twins and each baby has their own amniotic sac, your doctor will take two samples. Amniocentesis is a little trickier with twins than it is with single babies. Make sure your doctor or technician has experience doing amniocentesis with multiples.

In a small number of cases, the doctor has trouble removing enough fluid and will have to put the needle back in. If this doesn't work, you'll need to return another day for another amnio.

The entire procedure, including consultation, takes about 30 minutes. You may feel minor period-like cramping or discomfort during the amniocentesis or for a few hours after. Hospital staff will watch you for up to an hour afterward to make sure you're not having any side effects, such as heavy bleeding.

The results of the amniocentesis are generally available within 2 or 3 weeks. If you have not received the results within 3 weeks, call your health care provider. Some results that show whether your baby has a chromosomal condition like Down syndrome should be available within a few days after the amnio.

If the test shows a problem, you'll meet with a counselor, doctor, midwife, or geneticist to talk about the condition and your options. Doctors can sometimes treat certain birth defects, such as spina bifida, while your baby is still in the womb. Knowing about an issue can also help by increasing monitoring during pregnancy and helping you and your doctor to prepare.

After an amniocentesis, it is best to go home and relax for the rest of the day. You should not exercise or do any strenuous activity, lift anything over 20 pounds (including children), or have sex.

You may take acetaminophen every 4 hours to relieve discomfort. The day after the procedure, you may resume all of your normal activities unless otherwise directed by your doctor.

Call your doctor if you have a fever or have bleeding, a vaginal discharge, chills, contractions, or belly pain that is more severe than cramps.

Amniocentesis is a prenatal test used to gather information about a developing baby's health. The procedure, typically done between the 15th and 18th weeks of pregnancy, is around 99.4% accurate. It poses a slight chance of problems such as miscarriage, infection, or injury to you or your baby. 

Doctors suggest the test for people with higher odds of genetic diseases or certain birth defects based on family history, abnormal ultrasounds, or genetic test results. While amniocentesis can spot many conditions, it doesn't find all birth defects.