Prenatal Testing

Some tests, called indicated tests, are usually reserved for women who are considered at increased risk due to something her medical or genetic history or an abnormal result in a screening test. However, some indicated tests, such as ultrasound, are being used more often during pregnancy, regardless of whether the mother or fetus is at risk of health problems.

Prenatal tests include:

• Ultrasound

• Alpha-fetoprotein

• Contraction stress testing

• Non-stress testing

• Fetal motion count

• Amniocentesis

• Other screening tests

Ultrasound

Ultrasound, or sonogram, uses high frequency sound waves to form an image of the uterus, placenta, and fetus. This view into the uterus allows doctors to measure many details about the fetus, including:

• Growth of the fetus

• Abnormalities of the fetal structures such as heart, brain, limbs, kidneys, and stomach

• Birth defects

• Amount of amniotic fluid

• Location and development of the placenta

• Gestational age or duration of the pregnancy

The test can be performed almost any time during pregnancy. In the earliest months, it can be performed with a probe placed in the vagina. However, most are performed with a wand placed on the abdomen over the uterus.

Alpha-Fetoprotein (AFP)

Between 15 and 18 weeks, the doctor may perform a test to determine the level of alpha-fetoprotein (AFP) in a pregnant woman's blood. AFP is a substance normally produced by a growing fetus. In AFP testing, a blood sample taken from a vein in the arm is analyzed. If the test shows that the AFP level is higher or lower than normal, further tests will be done to confirm or rule out fetal problems.

High AFP levels will occur with twin pregnancies. Also, measuring AFP can help determine the presence of a type of birth defect called a neural tube defect. Neural tube defects are an abnormality in which the spinal cord or brain does not form properly. An increased level of AFP may be found in the blood of a woman whose fetus has a neural tube defect or an abdominal wall defect in formation. An AFP level that is lower than normal may be linked to an increased risk of Down syndrome.

Contraction Stress Testing

This test uses a fetal monitor to continuously record the baby's heart rate and uterine contraction on special paper. A decrease in the fetal heart rate in response to contractions of the uterus is a positive test result. This may involve the use of oxytocin or other means to cause uterine contraction. The test is performed when the fetus is believed to be at increased risk for stillbirth. A negative test suggests that the fetus is at low risk for stillbirth, whereas a positive test indicates that the fetus may be in danger.

Non-Stress Testing

In this procedure, the fetal monitor is attached to the mother's abdomen and records the fetal heart rate. The doctor listens for increases, or accelerations, in the fetal heart rate. If the baby is believed to be asleep, a buzzer is sounded to awaken the fetus. Like contraction stress testing, this test is also performed when the fetus is believed to be at increased risk for stillbirth, but it does not require uterine contractions. A flat fetal heart rate can indicate an increased risk of stillbirth.

Fetal Motion Count

There are a variety of ways in which this test is performed but all rely upon a sudden change in the number of fetal movements in a specified period of time. In most cases, the mother keeps track of the number of times she feels the baby move. Further tests will be performed if the mother notes a sudden decrease or absence of movements. Although some physicians use this procedure because it is easy to use and inexpensive, the appropriate role of fetal motion counting in prenatal care is controversial.

Amniocentesis

The amniotic fluid that surrounds the growing fetus can yield important information. Amniocentesis allows a small sample of this fluid to be collected for analysis. Using sonogram as a guide, doctors insert a long, thin needle through the abdominal wall and the wall of the uterus into the fluid cavity surrounding the fetus.

Amniocentesis may be done for many reasons:

• To identify genetic defects

• To test for fetal lung maturity

• To detect isoimmunization to Rh factor

Genetic amniocentesis. One of the most common reasons for amniocentesis is to identify genetic defects. Genes carry the master plan of a person's physical makeup. Because the amniotic fluid and the developing fetus are formed from the same cells, they share the same genetic makeup. Amniotic fluid can therefore be studied to see whether the fetus's chromosomes are normal. Amniocentesis may also be done as a follow-up procedure in the event of a positive AFP test. Since the results of genetic amniocentesis may influence parental decisions about whether or not to carry a pregnancy to term, it is performed early, usually during the fourth month.

Amniocentesis for fetal lung maturity. This test is performed if there is concern that the infant may be at risk for lung development problems. In order to remain open, the lungs require a substance known as surfactant, lack of which is amajor cause of lung problems in premature infants. Amniocentesis for fetal lung maturity testing is usually reserved for situations in which early delivery is desirable, but the lung maturity of the fetus is in question.

Amniocentesis for isoimmunization. Isoimmunization occurs when Rh or other antibodies from the mother cross the placenta and destroy red blood cells in the fetus, causing anemia. This destruction can be measured by testing the amniotic fluid for bilirubin, a reddish-yellow pigment formed mainly by the decomposition of hemoglobin in worn-out red blood cells. Amniocentesis for isoimmunization is usually performed at various intervals during the second half of pregnancy.

For more detailed information about Anemia, go to Anemia.

Other Screening Tests

A number of other screening tests provide further assurance that a baby is progressing normally.These tests include:

• Human chorionic gonadotropin (HCG). This substance can be measured in blood or urine samples taken from the mother. The most common reason to measure HCG is to diagnose pregnancy. However, physicians also use HCG testing to detect Down syndrome in a fetus.

• Estriol. Earlier in pregnancy, this measurement can be used in conjunction with maternal age, AFP, and HCG to help in the prediction of Down syndrome.