Amniocentesis is a prenatal test that allows healthcare practitioner to gather information about baby’s health from a sample of amniotic fluid. This is the fluid that surrounds baby in the uterus.
The most common reason to have an “amnio” is to determine whether a baby has certain genetic disorders or a chromosomal abnormality, such as Down syndrome.
Amniocentesis is usually done when a woman is between 14 and 16 weeks pregnant. The test is invasive and carries a small risk of miscarriage.
Before the start of the procedure, a local anesthetic can be given to the mother in order to relieve the pain felt during the insertion of the needle used to withdraw the fluid. After the local anesthetic is in effect, a needle is usually inserted through the mother’s abdominal wall, then through the wall of the uterus, and finally into the amniotic sac. With the aid of ultrasound-guidance, a physician punctures the sac in an area away from the fetus and extracts approximately 20ml of amniotic fluid. If used for prenatal genetic diagnosis, fetal cells are separated from the extracted sample. The cells are grown in a culture medium, then fixed and stained. Under a microscope the chromosomes are examined for abnormalities. The most common abnormalities detected are Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Turner syndrome (monosomy X). In regard to the fetus, the puncture seals and the amniotic sac replenishes the liquid over the next 24–48 hours.
INDICATIONS AND RESULT
Early in pregnancy, amniocentesis is used for diagnosis of chromosomal and other fetal problems such as:
• Down syndrome (trisomy 21)
• Trisomy 13
• Trisomy 18
• Fragile X
• Rare, inherited metabolic disorders
• Neural tube defects (anencephaly and spina bifida) by alpha-fetoprotein levels.
Amniocentesis can predict fetal lung maturity, which is inversely correlated to the risk of infant respiratory distress syndrome.
Amniocentesis can also be used to detect problems such as:
• Infection, in which amniocentesis can detect a decreased glucose level, a Gram stain showing bacteria or an abnormal differential count of white blood cells.
• Rh incompatibility
• Decompression of polyhydramnios