What is an amniocentesis?

An amniocentesis is a procedure used to obtain a small sample of the amniotic fluid that surrounds the fetus to diagnose chromosomal disorders and open neural tube defects (ONTDs) such as spina bifida. Testing is available for other genetic defects and disorders depending on the family history and availability of laboratory testing at the time of the procedure. An amniocentesis is generally offered to women between the 15th and 20th weeks of pregnancy who are at increased risk for chromosome abnormalities, or those who have had an abnormal maternal serum screening test, indicating an increased risk for a chromosomal abnormality or neural tube defect.

How is an amniocentesis performed?

An amniocentesis is a procedure that involves inserting a long, thin needle through the mother's abdomen into the amniotic sac to withdraw a small sample of the amniotic fluid for examination. The amniotic fluid contains cells shed by the fetus, which contain genetic information. Although specific details of each procedure vary slightly, generally, an amniocentesis follows this process:

  • The woman's abdomen is cleansed with an antiseptic.
  • The physician may/may not give a local anesthetic to numb the skin.
  • Ultrasound is used to help guide a hollow needle into the amniotic sac.
  • A small sample of fluid is withdrawn for laboratory analysis.
  • Strenuous activities should be avoided for 24 hours following an amniocentesis.
  • Women may feel some cramping during or after the amniocentesis.

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 patient's anatomy, sometimes the amniocentesis cannot be performed. The fluid is sent to a genetics laboratory so that the cells can grow and be analyzed. Alpha-fetoprotein, a protein made by the fetus that is present in the fluid, is also measured to rule out an open neural tube defect, such as spina bifida. Results are usually available in about 10 days to two weeks, depending on the laboratory.

Amniocentesis is usually recommended for women with abnormal screening tests, or for women who have a family history of inherited disorders. It is usually performed between the 15th and 20th weeks of pregnancy.

In amniocentesis, a thin needle is inserted through the mother’s abdomen into the sac around the baby under ultrasound guidance. A small amount of fluid is withdrawn and is sent to a laboratory. Cells are cultured and the baby’s chromosome makeup is reported in two weeks.

In addition, the amount of a substance called AFP is determined, which gives information about some structural defects such as spina bifida.

The amniocentesis procedure is brief and causes minimal discomfort to the patient. It is a relatively safe procedure with little risk to the pregnancy.

The North Shore-LIJ Women’s Health Maternal Fetal Medicine Specialists are experts in the diagnostic testing procedure known as amniocentesis.  For more information  call 516 562-4458, (genetics - 516 365-3996) for the Maternal Fetal Medicine Specialist at North Shore University Hospital, or call 718 470-7636 for the Maternal Fetal Medicine Specialist at LIJ Medical Center.