Healthy Pregnancy

What can an amniocentesis tell you?

Amniocentesis is sometimes called an “amnio” or a “tap.”

Between the 12 and 16th weeks of gestation (14-18 weeks pregnancy) you may be offered an amniocentesis. This test is performed by inserting a needle through your abdomen into the uterus and amniotic sac and collecting a sample of the fluid. Because of the risk of inserting a needle into the uterus, ultrasound is used to help guide the needle to a safe place.

This test is performed before 18 weeks pregnancy to check DNA for chromosomal problems such as Down Syndrome, Sickle Cell Anemia and any neural tube defects. Near the end of pregnancy it may be performed to determine if your baby’s lungs are mature or to check for Rh disease.

What you need to know

When the procedure was compared to Chorionic Villi Sampling (CVS), it was concluded that the amniocentesis is the safest and most accurate prenatal diagnostic procedure but should not be used before 15 weeks gestation (17 weeks pregnancy). If testing is needed before this, the transabdominal CVS is the best procedure followed by the transcervical CVS.

Amniocentesis does pose a risk of miscarrying. One half of one percent of babies who receive amniocentesis miscarry (1 in 200). There is also a risk your baby will be poked by the needle, however this rarely happens when ultrasound is used to guide the needle.

Because an amniocentesis collects DNA from your baby, it is 100% accurate at identifying the sex. However, it is never performed simply to know the sex. If you choose to have an amniocentesis, be sure to let your midwife know if you want to know the sex of your baby.

If you decide to have an amniocentesis, you may receive a local anesthetic to numb your abdomen, however this is not able to numb your uterus and you will feel the needle as it enters. Women rate the sensation anywhere from uncomfortable to painful. It is common to have cramping, leakage or discomfort around the needle site after this procedure. Mothers who are Rh negative will need an injection of Rhogam for this procedure.


Be sure to drink several glasses of water before an amniocentesis. In the 45 minute procedure you will experience:

  • A preliminary ultrasound exam
  • Antiseptic on your abdomen
  • A local anesthetic
  • A long hallow needle will be inserted into your uterine wall and amniotic sac through your abdomen – ultrasound will be used to help the insertion of the needle
  • A small fluid sample will be drawn
  • The needle will be removed and your abdomen will be bandaged
  • Results may take up to two weeks

Alfirevic Z, Sunberg K, Brigham S (2004). Amnicentesis and chorionic villi sampling for prenatal diagnosis (Cochrane Review). The Cochrane Library issue 2. Chichester, UK: John Wiley & Sons.

Jennifer (Author)