Amniocentesis

Overview

If you are at increased risk for chromosomal abnormalities your doctor may recommend a procedure called amniocentesis when you are between your 15th and 20th week of pregnancy. You may be at increased risk if you:

  • are age 35 or older
  • have had an abnormal maternal serum screening test
  • you have had a previous pregnancy or a child with a birth defect

Amniocentesis is a common prenatal test that involves taking a small sample of the amniotic fluid that surrounds the fetus and then culturing it in our laboratory. The test helps diagnose chromosomal disorders or open neural tube defects such as spina bifida. It can also be used in late pregnancy to assess fetal lung maturity. Other testing is available for other genetic defects and disorders depending on family history.

Preparation before the procedure

You will be asked to remove your clothing and wear a gown that we provide. There is a locker for your clothing and valuables. You will be able to have a partner, spouse or friend with you during the test.

How is it done?

The procedure involves inserting a long, thin needle through the abdomen into the amniotic sac to withdraw a small sample of the amniotic fluid for examination. Your body will replace the fluid that is withdrawn.

Although specific details of the procedure can vary slightly, generally an amniocentesis follows this process:

  • you will lie on a table and your abdomen will be cleansed with an antiseptic
  • you may be given a local anesthetic to numb the skin
  • warmed ultrasound gel is used
  • ultrasound will help guide the needle into the amniotic sac
  • a small sample of fluid is withdrawn for laboratory analysis.

Women with twins or other multiples need sampling from each amniotic sac, in order to study each baby. Depending on the position of the baby, placenta, amount of fluid or your anatomy, sometimes the amniocentesis cannot be performed.

The fluid is analyzed by highly skilled laboratory specialists at the University of Washington so that cells can grow and be analyzed.

What equipment is used?
A special needle is used for the safety of the fetus. Ultrasound technology, which is painless and will not harm you or your baby, is used to guide the needle.

How will it feel?

During the procedure you may feel a slight tingling or some pain when the needle is inserted. You may have some mild cramping during or after the procedure. Although it may take awhile for the ultrasound part of the test, the insertion and removal of the needle occurs quickly.

What are the risks and benefits?

The risks of amniocentesis include:

  • you may experience cramping, bleeding or leaking of amniotic fluid
  • there is also a slight risk of infection
  • the published risk of miscarriage or any other complication is about one in 200 after an amniocentesis in the second trimester of pregnancy, this is only slightly higher than the normal risk of miscarriage without an amniocentesis at this time in your pregnancy
  • some physicians will perform an amniocentesis between 12 and 15 weeks of pregnancy, this is called “early amniocentesis”
    • when this is the case, the risk for complications is between one in 100 at 12 weeks and one in 200 at 15 weeks

The benefits of amniocentesis include:

  • amniocentesis helps confirm a tentative diagnosis of an abnormality found with other testing
  • the procedure may also find that a fetus does not have a suspected abnormality
  • amniocentesis may provide information about an open neural tube defect (over 95 percent accurate; closed or small defects may show a normal result)
  • diagnosis of other defects depending on family history

Who interprets results and how will I get them?

Highly skilled laboratory specialists will interpret the results and discuss them with your doctor. Your doctor will call you, usually within ten days, to let you know the test results.

After the procedure

  • the fetal heart rate is checked
  • you may feel some cramping during or after the amniocentesis
  • you should avoid strenuous activities for 24 hours following the procedure
  • call your doctor if you experience bleeding, leaking fluid or strong cramping