Testing 1-2-3

The prenatal tests you may receive and what they mean

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It goes with the territory: When you’re pregnant, you can’t help but worry about the health of your baby. Fortunately, there are a host of prenatal tests that can help ease your fears and make even a healthy pregnancy less stressful. Following is a rundown of the tests you’re most likely to undergo; see the chart at right for detailed information.

Screening tests

During pregnancy, women undergo several screening tests to rule out preexisting health problems and potential problems with the fetus. Most women’s prenatal testing begins with the first visit to the obstetrician or midwife, at which blood and urine tests will be done, as well as a Pap smear, if necessary. Several tests will be performed at later visits to track your pregnancy and monitor your baby’s health. For example, at each visit, your blood pressure will be measured and a sample of your urine examined. The doctor will note your weight and may listen to the baby’s heartbeat and take external measurements of your uterus to monitor the baby’s growth. At some point, an ultrasound may be performed to determine your due date or check your baby’s development.

One of the newer tests is cystic fibrosis screening, which determines whether a woman or her partner is a carrier of the disease. While the test is available to all pregnant women, your doctor or midwife will probably recommend it if you are of European or Ashkenazi Jewish descent. If you are interested in having the test done, ask your doctor. It is usually done via a blood sample in early pregnancy.

Other screening tests include the maternal serum alpha-fetoprotein (MSAFP or AFP), which is performed at about 15 to 20 weeks. This blood test helps screen for such conditions as neural-tube defects and Down syndrome. The results of the AFP test are not conclusive, however; abnormal results simply indicate that more testing is necessary.

Diagnostic tests

If any of your screening tests produces abnormal results, if you have a personal or family history of birth defects or if you will be over the age of 35 when you deliver, you may undergo certain diagnostic tests to rule out problems with the fetus. The two most common are amniocentesis and chorionic villus sampling (CVS).

Amniocentesis can detect all known chromosomal “errors,” such as Down syndrome, as well as many genetic disorders. It is typically performed between the 15th and 20th week of pregnancy; a needle is inserted through the abdomen into the uterus to withdraw a sample of amniotic fluid. Because it is an invasive test, there is a 1 in 200–400 risk of miscarriage following the procedure.

CVS, which also can detect chromosomal or genetic birth defects, has a slightly higher risk of miscarriage (1 in 100–200), but it can be performed as soon as 10 weeks after conception. With this procedure, a needle is inserted through the abdomen or a catheter is inserted through the vagina to take a sample of the outer fetal membrane. If amniocentesis or CVS testing is indicated, you may want to consult a genetic counselor or your religious adviser to discuss the kinds of decisions such testing may lead to. For more information about prenatal testing, contact the March of Dimes at 888-663-4637, www.marchofdimes.com; or the American College of Obstetricians and Gynecologists at 202-863-2518, www.acog.org.

PRENATAL TESTS AT A GLANCE

Test Blood type and Rh factor

Who needs it, when is it performed? All pregnant women; at first visit.

What's Involved? A blood sample is drawn and examined.

What does it reveal? Mother’s blood type and whether she is Rh-negative.

Potential harm to fetus? None.

What's next? Follow-up: If Rh-negative, Rh immune globulin is given at 28 weeks.

Test Pap smear/ gonorrhea culture

Who needs it, when is it performed? Women who have not had a recent Pap smear or who are at risk for having gonorrhea; at first visit.

What's Involved? Cervical/vaginal secretions are examined.

What does it reveal? Preexisting medical conditions such as cervical cancer and vaginal infections.

Potential harm to fetus? None.

What's next? Follow-up: Depends on findings.

Test Hematocrit/ hemoglobin

Who needs it, when is it performed? All; early in pregnancy and at 32–36 weeks.

What's Involved? A blood sample is drawn and examined.

What does it reveal? Can indicate anemia.

Potential harm to fetus? None.

What's next? Follow-up: If anemic, extra iron will be prescribed

Test Cystic fibrosis screening

Who needs it, when is it performed? Women of certain races and ethnicities who are at risk for carrying the disease; early in pregnancy.

What's Involved? A blood (or, occasionally, saliva) sample is drawn and examined.

What does it reveal? Whether mother is a carrier.

Potential harm to fetus? None.

What's next? Follow-up: If mother is a carrier, the father may be tested; both may be referred for genetic counseling.

Test Screening for illnesses (HIV, hepatitis B, etc.)

Who needs it, when is it performed? All; early in pregnancy. Repeated if mother is exposed to an illness.

What's Involved? A blood sample is drawn and examined.

What does it reveal? Whether mother has been exposed to a disease or illness.

Potential harm to fetus? None.

What's next? Follow-up: Mother is treated to prevent transmission to fetus at birth. Test Blood pressure

Who needs it, when is it performed? All; at every visit.

What's Involved? Blood pressure is measured.

What does it reveal? A sudden rise can signal complications.

Potential harm to fetus? None.

What's next? Follow-up: Additional testing may be needed.

Test Urinalysis

Who needs it, when is it performed? All; at every visit.

What's Involved? A urine sample is collected and examined.

What does it reveal? Protein can signal preeclampsia; glucose can signal gestational diabetes.

Potential harm to fetus? None.

What's next? Follow-up: High levels of protein or glucose require further testing (see blood-sugar test)

Test Maternal serum alphafetoprotein

Who needs it, when is it performed? All; at 15–20 weeks.

What's Involved? Blood is drawn and examined for alpha-fetoprotein, a substance produced by the fetus.

What does it reveal? Abnormal levels can indicate neural-tube defects, Down syndrome or a multiple pregnancy.

Potential harm to fetus? None.

What's next? Follow-up: f abnormal levels are found, ultrasound and/or amniocentesis may be suggested to rule out defects or chromosomal errors.

Test Ultrasound

Who needs it, when is it performed? Performed as needed to gauge due date or gestational age.

What's Involved? Also used during amniocentesis and CVS. A device placed on the abdomen or in the vagina creates an image of the fetus.

What does it reveal? Age and position of fetus, growth rate, placement of placenta and visible defects.

Potential harm to fetus? None.

What's next? Follow-up: Depends on findings; may indicate need for further testing.

Test Amniocentesis

Who needs it, when is it performed? Women with a history of birth defects or who will be over 35 at term (or possibly if abnormal AFP results are found); at 15–20 weeks.

What's Involved? A needle is inserted through the abdomen and into the uterus; a sample of amniotic fluid is withdrawn.

What does it reveal? Chromosomal or genetic birth defects such as Down syndrome or spina bifida. Accuracy rate is 99.4%

Potential harm to fetus? Slightly increased risk of miscarriage (1 in 200–400).

What's next? Follow-up: Depends on findings; genetic counseling may be advised if an abnormality is found.

Test Chorionic villus sampling

Who needs it, when is it performed? Women with a history of birth defects or who will be over 35 at term; at 10–12 weeks.

What's Involved? A needle is inserted through the abdomen and uterus (or a catheter is inserted through the vagina and cervix) to sample the outer fetal membrane.

What does it reveal? Chromosomal or genetic birth defects such as Down syndrome or spina bifida. Accuracy rate is about 98%

Potential harm to fetus? Slightly increased risk of miscarriage (1 in 100–200) and damage to fetus.

What's next? Follow-up: Depends on findings; genetic counseling may be advised if an abnormality is found.

Test Blood sugar

Who needs it, when is it performed? Most women; at 24–28 weeks or when glucose is found in the urine.

What's Involved? A blood sample is drawn after the mother drinks a sugary mixture.

What does it reveal? Glucose level in the blood may indicate gestational diabetes.

Potential harm to fetus? None.

What's next? Follow-up: A change in diet and/or insulin treatment may be prescribed.

Test Group B Strep

Who needs it, when is it performed? All women; at 35–37 weeks.

What's Involved? Vagina and perineum are swabbed and sample is examined.

What does it reveal? Presence of potentially deadly bacteria that can be passed to baby at birth.

Potential harm to fetus? None.

What's next? Follow-up: Antibiotics are given during labor if bacteria are found.

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