Amniocentesis

Amniocentesis is a medical procedure in which a long, hollow needle is used to draw amniotic fluid from a pregnant woman’s uterus in order to test for various abnormalities that may affect the health of her baby.  Many women find this procedure terrifying, but it is sometimes necessary in order to determine a fetus’s chances for survival and a healthy life, and to help prospective parents plan for the possibility that their child might have special needs. Amniocentesis is often one of the factors that must be weighed by a woman who is considering termination of her pregnancy if there is a diagnosed birth defect.

What is Amniotic Fluid?

Amniotic fluid is the bath in which a developing baby grows. It protects the fetus from injury and allows for proper organ development. The fetus moves in the fluid, breathes it in and out, swallows it, and urinates into it, allowing for proper development of the limbs and bodily functions. Early in gestation, the amniotic fluid is also a major supplier of nutrients to the developing fetus, but this function diminishes as the fetal circulation develops. The fluid also contains hormones and provides some antibacterial properties. Amniotic fluid is much more than just water!

Reasons for Amniocentesis

There are two common situations where an amniocentesis test might be indicated —genetic amniocentesis and maturity amniocentesis—and they are performed for different reasons.

Genetic amniocentesis is generally not done until after the 17th week of pregnancy or later. Early amniocentesis performed at week 16 or sooner has been shown to be associated with a higher rate of pregnancy loss and is rarely done. If fetal DNA is needed that early, experts prefer CVS (chorionic villous sampling — a more invasive procedure than amniocentesis).

The purpose of genetic amniocentesis is to determine whether your baby is likely to suffer from certain genetic abnormalities or birth defects. Genetic amniocentesis can be used to test for a variety of conditions, including:

  • Spinal bifida
  • Down syndrome, also called Trisomy 21. This is when there is an extra chromosome (the number 21 chromosome, out of 23 total). Amniocentesis predicts this condition with 99.9% accuracy.
  • Genetic conditions such as Edwards syndrome (also known as Trisomy 18)
  • Other chromosomal abnormalities, such as a missing chromosome, or an extra one. This includes a missing X chromosome (Turner’s Syndrome), an extra chromosome number 13 (Patau Syndrome), and many others, all of which are rare.

Maturity amniocentesis is performed in order to make sure the baby’s lungs are mature enough to function well outside the womb. A baby’s lungs are not likely to be fully developed before the 34th week of pregnancy, so this procedure is performed only when other medical considerations (such as preterm premature rupture of membranes, for example) have forced doctors to consider delivering the baby early, either via C-section or by inducing labor.

Doctors generally recommend amniocentesis for women who fit one or more of the following descriptions:

  • Over the age of 35
  • Have had test results suggesting that the fetus may be at risk for some genetic abnormality, such as an abnormal finding seen during an ultrasound or an abnormal prenatal screening blood test
  • Have a family history of genetic abnormalities, or a partner with such a family history
  • Have previously given birth to babies with birth defects—especially Down syndrome or any type of neural tube defect.

Other Reasons for Amniocentesis

Amniocentesis may also be performed to test for the presence of infection, or to drain some of the amniotic fluid from the uterus in cases of polyhydramnios, a condition in which there is an excess of amniotic fluid. In some cases it is used to determine the severity of Rh disease, a condition caused by incompatibility between maternal and fetal blood types.

Risks of Amniocentesis

Like any medical procedure, amniocentesis carries some risks. These include:

  • Fetal injury from the needle. Although it is rare, it is not impossible for the baby to suddenly move an arm or leg into the path of the needle.
  • Miscarriage. The rate of miscarriage caused by amniocentesis is small—roughly one in 400—but it is not completely insignificant. You must remember, however, that your doctor will not recommend amniocentesis on a whim, and the benefits generally outweigh the risks.
  • Amniotic fluid leak—it is possible, in rare cases, for amniotic fluid to leak out of the vagina following amniocentesis. This can potentially cause oligohydramnios, a deficiency in amniotic fluid that can lead to other pregnancy complications such as premature birth, infection, or improper fetal limb or lung development.
  • Infection transmission—there is a small chance, if you have an infection such as hepatitis or HIV, that it could be transferred to your baby.

A doctor may discourage amniocentesis in women whose pregnancies are affected by placenta previa or placental abruption, or who have experienced premature labor with previous pregnancies. An incompetent cervix is also a reason to think twice before undergoing amniocentesis.

What to Expect

Genetic amniocentesis can be done in a doctor’s office; there is no need for you to go to a hospital. Blood samples will be taken before the tests, and if your blood type is Rh negative you may be given a shot of a medication known as RhoGAM to protect you in the unlikely event that the procedure causes some of your baby’s blood to enter your own bloodstream, which can trigger an undesirable immune-system reaction.

After you lie down on the table, the doctor will first perform an ultrasound in order to determine your baby’s position and avoid injuring him or her. A topical anesthetic may then be applied to your belly or injected, so that you do not feel any pain (your experience may vary; not all doctors use anesthetic for amniocentesis, and women who have the procedure done without it report only mild discomfort). Using ultrasound guidance, the doctor will then insert a long, thin, hollow needle into your belly. This needle will go through your uterus and into the amniotic sac, withdrawing about four teaspoons of amniotic fluid. This process takes about 20 to 30 minutes.

You should expect to feel very few ill effects after the amniocentesis procedure, but you should contact your doctor immediately if you experience:

  • Bright red vaginal bleeding
  • Persistent vaginal fluid leakage or discharge
  • Abdominal cramping or pain lasting more than a few hours
  • Inflammation at the site of needle insertion
  • Fever greater than 100 degrees

Results

Usually results take about seven to ten days. This is because fetal skin cells from the amniotic fluid have to be grown in culture before they can be tested for DNA. Chemical tests on the fluid are quicker, requiring maybe a day or two. There is a special type of DNA test using fluorescent probes that can provide a preliminary DNA result in 48–72 hours. This is called the “FISH” test (fluorescence in-situ hybridization). The FISH test can identify up to 95 percent of all potential chromosomal conditions, including all of the ones described earlier in this article.

Talking to Your Doctor

Here are some questions you can ask your doctor about amniocentesis:

  • Given my genetic history and my spouse’s, do you recommend that I have amniocentesis?
  • Will it hurt?
  • How long will it take for me to get results? Is there a way to get them sooner?
  • Does my age put my baby or me at additional risk?
  • Is there a chance that my amniocentesis could cause a miscarriage?
  • No, really, tell me the truth—will it hurt? That’s the biggest needle I’ve ever seen!
  • Will this test tell us with 100 percent certainty whether my baby will suffer any kind of birth defect?
  • I have heard about a new blood test with which they can get DNA from my baby by drawing my blood. Can I do this blood test and then avoid having to do the amniocentesis?

This page was last updated on 06/2017
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