Amniocentesis involves the collection and analysis of an amniotic fluid sample.
Before an amniocentesis (and often during the entire procedure), a fetal ultrasound test is done to locate the fetus, the placenta, and a large pocket of amniotic fluid. A long, thin needle is inserted through the abdomen into the uterus, avoiding the fetus and placenta, and a small amount [15 mL (1 Tbsp) to 30 mL (2 Tbsp)] of amniotic fluid is collected through the needle.
See a picture of an amniocentesis.
Amniotic fluid contains cells that have been shed by the developing fetus. These can be examined for chromosomal defects that cause conditions such as Down syndrome and cystic fibrosis. Amniotic fluid can also be used to identify the sex of the fetus.
Amniotic fluid can be tested for a fetus's Rh factor. (Samples from the mother's and father's blood are tested and compared.) If the fetus is Rh-negative, no further testing or treatment is necessary during the current pregnancy.
Chemicals in the amniotic fluid can show whether an Rh-positive fetus is being harmed by the mother's antibodies to the Rh factor.
Bilirubin is naturally found in amniotic fluid. During a normal pregnancy, the bilirubin level increases until 23 to 25 weeks of pregnancy, and then decreases throughout the remainder of the pregnancy. If fetal blood cells are being attacked by the mother's immune system, the bilirubin level continues to increase throughout the pregnancy. After the 20th week of pregnancy, bilirubin measurements are accurate enough to guide further testing or treatment.
Amniotic fluid can also be tested for substances from the fetus's lungs. Test results show how mature the lungs are. This is important information if the baby has to be born early.
Amniocentesis may be done to:
When maternal Rh antibody levels are too high, an amniocentesis may be done. The amount of bilirubin found in the amniotic fluid is used to predict the level of fetal harm that is occurring from Rh sensitization.
The results of amniocentesis may guide treatment for Rh sensitization. The level of fetal lung maturity may help plan the timing of delivery.
Amniocentesis carries a slight risk of injuring the fetus, starting labour, or introducing an infection into the uterus.
For checking anemia in a fetus, Doppler ultrasound can give you the same information as amniocentesis, without the risks.2 Doppler ultrasound can only be done by a well-trained provider and is not available everywhere.
A simple test of the mother's blood, for information about her fetus's blood type, may be available in the future.
Amniocentesis is less sensitive than fetal blood sampling (FBS). However, because it is less risky than FBS, amniocentesis is the preferred test for detecting mild to moderate Rh disease.
Normal results from amniocentesis do not guarantee that the baby will be healthy.
Amniocentesis may cause mixing of the mother's and fetus's blood. Therefore, unsensitized Rh-negative women are given Rh immune globulin after amniocentesis to prevent sensitization.
- Caughey AB, et al. (2006). Chorionic villus sampling compared with amniocentesis and the difference in the rate of pregnancy loss. Obstetrics and Gynecology, 108(3): 612–616.
- American College of Obstetricians and Gynecologists (2006, reaffirmed 2008). Management of alloimunization during pregnancy. ACOG Practice Bulletin No. 75. Obstetrics and Gynecology, 108(20): 457–464.
- Seeds JW (2004). Diagnostic mid trimester amniocentesis: How safe? American Journal of Obstetrics and Gynecology, 191: 608–616.
Last Revised: January 26, 2012
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