Amniocentesis is the withdrawal through a hollow needle of a sample of the fluid, called amniotic fluid, surrounding a fetus in the uterus by piercing the amniotic
sac through the abdominal wall, under direct ultrasound guidance. As the amniotic fluid contains cells from the fetus (mostly shed
from the skin), cell cultures enable chromosome patterns to be studied so that prenatal diagnosis of chromosomal abnormalities
(such as Down's syndrome) can be made. Certain metabolic errors and other
abnormalities, such as spina bifida,
can also be diagnosed prenatally from analysis of the cells or of the fluid.
Amniocentesis is usually performed at any time from the 15th completed week of gestation.
How the test is performed
Amniocentesis is done on an outpatient basis: no stay in hospital is need. The exact location of the baby is found, usually by performing a pregnancy ultrasound. An area of skin on the mother's belly area is cleaned. A numbing medication (anesthetic) may be applied to the skin, or a local anesthetic may be injected into the skin. A long, thin needle is inserted through the abdomen and into the womb (uterus). A small amount of fluid is taken from the fluid-filled sac that surrounds the baby.
How to prepare for the test
The patient's bladder must be full for the ultrasound. There are no food or drink restrictions. A blood sample may be needed to determine your blood type and Rh factor. An injection of a medication called Rhogam may be given if the patient's blood is Rh negative.
How the test feels
If an anesthetic is used, a sharp, stinging sensation may be felt for a
few seconds. When the needle enters the amniotic sac, a sharp pain may be
felt lasting a few seconds. Some women feel pressure in the lower abdomen
when the fluid is pulled out. After the procedure, you may have some minor
Why the test is performed
The test can find chromosome problems such as:
Later on in a pregnancy, the test may be used to find problems such as:
This test is also sometimes done later in pregnancy to determine whether the baby's lungs are developed if there is a condition that requires early delivery.