Amniocentesis for Rh sensitization during pregnancy

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Amniocentesis for Rh sensitization during pregnancy

Examination Overview

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.

Rh disease

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.

Fetal lung maturity

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.

Why It Is Done

Amniocentesis may be done to:

  • Learn the fetal blood type and Rh factor. An Rh-negative fetus is not at risk for Rh disease, even if the mother is Rh-sensitized.
  • Check fetal health when the mother is sensitized to the Rh factor. (Doppler ultrasound, when done by a well-trained provider, can give you the same anemia information without the risks of amniocentesis.2 Doppler ultrasound is not available everywhere.)
  • Learn whether and when fetal blood sampling is needed.
  • Learn whether fetal lungs are mature if the fetus needs to be delivered earlier than 37 weeks (preterm birth).


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.

  • Amniocentesis is repeated every 2 to 4 weeks if the fetus is mildly affected. The fetus is usually delivered close to term.
  • Amniocentesis is repeated every 1 to 2 weeks if the fetus is being moderately affected. The fetus is usually delivered earlier than 38 weeks and may need a blood transfusion after birth.
  • Amniocentesis may be repeated every week if the fetus is being severely affected. The fetus may need a blood transfusion before birth and is usually delivered early.

The results of amniocentesis may guide treatment for Rh sensitization. The level of fetal lung maturity may help plan the timing of delivery.

  • If the bilirubin levels are very high but the fetus is less than 32 weeks' gestation, a fetal blood transfusion before birth may be done to keep the fetus healthy until delivery is possible.
  • If bilirubin levels are very high, and the fetus is older than 32 weeks' gestation but the fetus's lungs are still immature, medicine may be given to speed up fetal lung development. Delivery is ideally done 2 days later.
  • If fetal lungs appear to be mature, then delivery does not need to be delayed.

What To Think About

Amniocentesis carries a slight risk of injuring the fetus, starting labour, or introducing an infection into the uterus.

When amniocentesis is done by a highly trained provider, the risk for miscarriage may be as low as 1 in 400, according to one study.1 Some studies have shown higher risks, between 2 and 4 in 400.3

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.

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  1. 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.
  2. 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.
  3. Seeds JW (2004). Diagnostic mid trimester amniocentesis: How safe? American Journal of Obstetrics and Gynecology, 191: 608–616.


By Healthwise Staff
Primary Medical Reviewer Sarah Marshall, MD - Family Medicine
Specialist Medical Reviewer William Gilbert, MD - Maternal and Fetal Medicine
Specialist Medical Reviewer Donald Sproule, MD, CM, CCFP, FCFP - Family Medicine
Last Revised January 26, 2010

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