Amniocentesis for Rh sensitization during pregnancy

Exam 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 [1 to 2 Tbsp (29.57 mL)] of amniotic fluid is collected through the needle.

See an illustration 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

The chemicals lecithin and sphingomyelin are produced by the fetus's lungs and are found in the amniotic fluid. Their amounts are measured and their levels are compared as a ratio (L/S ratio). This number gives an indication of fetal lung maturity and ability to breathe independently if an early (preterm) delivery is necessary.

Another estimate of fetal maturity is the amount of phosphatidylglycerol (PG) in the amniotic fluid. This test is usually done during the same amniocentesis but is determined prior to the L/S ratios, especially for pregnant women who have diabetes. When tested in a diabetic woman, the L/S ratio is more likely to show that the fetus's lungs are mature when they are not.

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, even if the mother is Rh-sensitized.
• Check fetal health when the mother is sensitized to the Rh factor. (Doppler ultrasound can give you the same information without the risks of amniocentesis.¹)
• 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).

Results

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 that is shown by the L/S ratio may help plan the timing of delivery.

• If the bilirubin levels are very high but the fetus is less than 32 weeks' gestation, a 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, medication 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 labor, 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.² 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.¹

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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Author:	Kathe Gallagher, MSW	Last Updated November 10, 2005
Medical Review:	Patrice Burgess, MD - Family Medicine Gregory A L Davies, MD, FRCSC, FACOG - Maternal-Fetal Medicine Kirtly Jones, MD - Obstetrics and Gynecology
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