A User's Guide to Prenatal Tests

A User's Guide to Prenatal Tests

From the WebMD Archives

I must confess -- before I became pregnant, I was a medical wimp. A finger prick before my wedding day and annual checkups and pap smears were the extent of my relationship with doctors, and I liked it that way. But my first prenatal exam changed all that.

I soon discovered that no matter how young or healthy, moms-to-be face a variety of prenatal tests to monitor the health of their babies and their pregnancies. In most cases, these tests offer reassurance that everything is going smoothly, and many of them are meant to spot problems, such as iron deficiency or diabetes, that can be treated before complications occur.

Other tests, particularly those that are used to detect genetic problems, such as Down syndrome, cystic fibrosis or spina bifida, can present some tough choices and worries for parents-to-be. The initial screening tests are not meant to be conclusive. They are intended to determine if you are at higher-than-usual risk and may call for additional diagnostic tests that carry some risk to the fetus, although the odds of a healthy baby are still high -- only about 2% to 3% of all babies are born with a genetic defect.

To decide which of these tests are right for you, it's important to carefully discuss with your doctor or midwife what these tests are supposed to measure, how reliable they are, the potential risks, and your options and plans if the results hold bad news, says Dr. Michael Mennuti, chairman of obstetrics at the University of Pennsylvania School of Medicine.

"If a woman and her partner say they wouldn't do anything differently if the test showed a problem, then the value of a diagnostic test like amniocentesis or chorionic villi sampling (CVS) isn't there," Dr. Mennuti says. The test might be worthwhile, however, if a couple would consider terminating the pregnancy or simply wants to prepare for a child with special needs, he adds.

Here's a look at the most common prenatal tests you might undergo over the next nine months. Some tests, such as urine or blood tests, might be repeated during your pregnancy, as will routine blood pressure screening. Others, such as CVS and amniocentesis, may not be offered unless your age or other factors indicate that you or your baby are at higher risk for certain conditions or diseases.

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First Trimester Tests

Here are some tests you may undergo during the first trimester of your pregnancy:

Blood tests: During one of your initial exams, your doctor or midwife will identify your blood type and Rh factor, measure the level of iron in your blood, check for immunity to rubella (German measles), and test for hepatitis B, syphilis and HIV. Depending on racial, ethnic or family background, you may be offered tests and genetic counseling to assess risks for diseases such as Tay-Sachs, canavan, cystic fibrosis, thalassemia and sickle-cell anemia (if these weren't done at a preconception visit).

Urine tests: You'll also be asked early on for a urine sample so that your doctor or midwife can look for signs of kidney infection and, if necessary, to confirm your pregnancy by measuring the level of hCG, a hormone secreted by the placenta. (A blood test to confirm pregnancy may be used instead.) Urine samples will then be collected regularly to detect glucose (a sign of diabetes) and albumin (a protein that indicates pre-eclampsia, which is pregnancy-induced high blood pressure).

Cervical swabs: You'll also have a Pap smear to check for cervical cancer, and vaginal swabs may be taken to test for chlamydia, gonorrhea and bacterial vaginosis, an infection that can cause preterm birth. Treatment of these conditions will prevent complications for your newborn.

Chorionic villi sampling (CVS): If you're 35 or older or have a family history of certain diseases, you'll be offered this optional, invasive test between 10 and 12 weeks, which can detect many genetic defects, such as Down syndrome, sickle-cell, cystic fibrosis, hemophilia, Huntington's chorea and muscular dystrophy. The procedure involves either threading a tiny catheter through your cervix or inserting a needle into your abdomen to obtain a tissue sample from the placenta. The procedure carries a 1% risk of inducing miscarriage and is about 99% accurate in ruling out certain chromosomal birth defects. But in contrast to amniocentesis, it does not help in detecting open spine disorders, such as spina bifida and anencephaly, or abdominal wall defects.

A promising new procedure that combines a blood test with an ultrasound to screen for Down syndrome may be available between 10 and 14 weeks. The results of a blood test that measures either hCG and PAP-A (pregnancy-associated protein A) in maternal blood are used with an ultrasound measurement of the fetus' skin at the back of its neck (called nuchal-translucency). The procedure may be able to pick up a substantial portion of Down syndrome cases and other genetic diseases, says Dr. John Larsen, professor of obstetrics, gynecology and genetics at George Washington University. However, as with all screening methods, a more invasive diagnostic technique like CVS is used to follow up a positive screen.

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Second Trimester Tests

Here are the prenatal tests you may face in the second trimester:

Maternal serum alpha-fetoprotein (MSAFP) and multiple marker screening: One or the other is offered routinely at 15 to 18 weeks. The MSAFP test measures the level of alpha-fetoprotein, a protein produced by the fetus. Abnormal levels indicate the possibility (not existence) of Down syndrome or a neural-tube defect such as spina bifida, which can then be confirmed by ultrasound or amniocentesis.

When blood drawn for the MSAFP test is also used to check levels of the hormones estriol and hCG, it's called a multiple-marker screening. The multiple marker test significantly boosts the detection rates for Down syndrome. The test picks up about 80% of neural tube defects and 70% of Down syndrome, but many women will have a false-positive screening. About 3% to 5% of women who have the screening test will have an abnormal reading, but only about 10% of those women will have a child with a genetic problem.

Ultrasounds: A sonogram may be offered between 18 and 20 weeks for a variety of reasons, including verifying a due date, checking for multiple fetuses, investigating complications such as placenta previa (a low-lying placenta) or slow fetal growth, or detecting malformations like cleft palate. During the procedure, a device is moved across the abdomen that transmits sound waves to create an image of the uterus and fetus on a computer monitor. New three-dimensional sonograms provide an even clearer picture of your baby, but they aren't available everywhere and it's not clear whether they're any better than two-dimensional pictures in contributing to a healthy pregnancy or birth.

Glucose screening: Typically done at about 25 to 28 weeks, this is a routine test for pregnancy-induced diabetes, which can result in overly large babies, difficult deliveries and health problems for you and your baby. This test measures your blood-sugar level an hour after you've had a glass of soda. If the reading is high, which happens about 20% of the time, you'll take a more sensitive glucose-tolerance test, in which you drink a glucose solution on an empty stomach and have your blood drawn once every hour for three hours.

Amniocentesis: This optional diagnostic test is usually performed between 15 and 18 weeks (but can be done earlier) for women who are 35 or older, have a higher-than-usual risk of genetic disorders, or whose AFP or multiple-marker screen test results were suspicious. The procedure is done by inserting a needle through the abdomen into the amniotic sac and withdrawing fluid that contains fetal cells. Analysis can detect neural tube defects and genetic disorders. The miscarriage rate varies depending on the experience of the physician, averaging about 0.2% to 0.5% at 15 weeks and 2.2% at 11 to 14 weeks, but the test can detect 99% of neural-tube defects and almost 100% of certain genetic abnormalities.

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