What is amniocentesis?

Amniocentesis is a procedure used to obtain a small sample of the amniotic fluid that surrounds the fetus during pregnancy. In addition to various enzymes, proteins, hormones, and other substances, the amniotic fluid contains cells shed by the fetus. These cells contain genetic information that can be used to diagnose chromosomal disorders and open neural tube defects, such as spina bifida. Amniotic fluid also contains other substances that can be measured or tested for certain conditions.

Who should have an amniocentesis?

An amniocentesis is generally offered to women between the 15th and 20th weeks of pregnancy who are at increased risk for chromosome abnormalities. This includes women who are over 35 years of age at delivery, or those who have had an abnormal blood screening test indicating an increased risk for a chromosomal abnormality or neural tube defect. Amniocentesis helps confirm a tentative diagnosis of an abnormality previously found with other testing. It may also find that a fetus does not have the abnormality that was suspected.

What should I expect from the procedure?

An amniocentesis may be performed on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your physician’s practices. Generally, an amniocentesis follows this process:

  1. You will be asked to undress completely, or from the waist down, and put on a hospital gown.
  2. You will be asked to lie down on an examination table and place your hands behind your head.
  3. Your vital signs (blood pressure, heart rate, and breathing rate) will be checked.
  4. An ultrasound will be performed to check the fetal heart rate, the position of the placenta, fetus, and umbilical cord, and to locate a pocket of amniotic fluid.
  5. Your abdomen will be cleansed with an antiseptic. You will be instructed not to touch the sterile area on your abdomen during the procedure.
  6. You will feel a needle stick if a local anesthetic is injected. This may cause a brief stinging sensation.
  7. Ultrasound will be used to help guide a long, thin, hollow needle through your abdomen, into the uterus and the amniotic sac. This may be slightly painful. You may feel some cramping as the needle enters the uterus.
  8. The physician will withdraw a small amount of amniotic fluid into a syringe. The amount depends upon the type of testing that will be performed, but usually no more than an ounce is removed. You may feel a tugging or pulling sensation as the fluid is removed.
  9. The fluid will be placed into a special light-protected container.
  10. The needle will be removed.
  11. An adhesive bandage will be placed over the needle site.
  12. The fetus’ heart rate and your vital signs will be reassessed.
  13. If you are Rh negative, you may be given Rhogam, a specially developed blood product that can prevent an Rh negative mother's antibodies from reacting to Rh positive fetal cells.
  14. The amniotic fluid will be sent to the lab for examination.

Are there any risks?

As with any invasive procedure, complications may occur. Some possible complications may include, but are not limited to cramping, bleeding, or leaking of amniotic fluid from the puncture site or the vagina, infection, miscarriage, or preterm labor.

The risk of miscarriage is generally considered to be less than one percent 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 pregnancy.

What happens after the procedure?

You and your fetus will be monitored for a time after the procedure. Your vital signs and the fetal heart rate will be checked periodically for an hour or longer. Depending on test results, counseling with a genetics specialist may be recommended. You may feel some cramping during or after the amniocentesis. If you feel lightheaded, dizzy, or nauseated, notify the nurse. You may be instructed to rest on your left side. You should rest at home and avoid strenuous activities for at least 24 hours, or as directed by your physician. Notify your physician to report any of the following:

  • Any bleeding or leaking of amniotic fluid from the needle puncture site or the vagina
  • Fever and/or chills
  • Severe abdominal pain and/or cramping
  • Changes in the activity level of your fetus (if you are past 20 weeks of pregnancy)