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Common Prenatal Tests 101

Most parents-to-be can't help but have occasional worries about their baby's health. Fortunately, the vast majority of children born in this country are healthy. Prenatal tests are available to put your mind at ease -- though, ironically, thinking about the test results is part of what makes expectant parents nervous. Remember that the results of prenatal tests usually provide reassurance that your baby is thriving. Learn about four common procedures and why your doctor will recommend at least one of them.

What does it test for?

Many doctors do an ultrasound (which produces a picture called a sonogram, so the terms are interchangeable) to verify the age of the fetus, see if you're carrying more than one baby, and check for birth defects.

When is it done?

It can be performed at any stage of pregnancy.

How is it performed?

A doctor or ultrasound technician puts a jelly on your abdomen and then presses a wand on it. Sound waves from the wand produce a picture of your uterus on the screen. It doesn't hurt, but the sonogram may be confusing to look at -- your doctor can explain what she sees. The doctor or technician looks your baby over from head to toe, checking, for instance, for signs of a healthy spine and heart. She also checks the placenta, calculates the baby's approximate weight, and examines the baby's position to make sure everything looks healthy.

What if the results are abnormal?

If any problems are suspected, or sometimes just for the sake of extra caution, you may get a more detailed ultrasound exam, called a targeted, comprehensive, or level II exam. It may use more sophisticated ultrasound equipment and will take longer, often more than 30 minutes.

What does it test for?

Maternal blood tests show whether the baby is at higher-than-average risk for certain birth defects of the brain and spine (called neural tube defects) and Down syndrome (a chromosomal birth defect).

When is it done?

It's usually performed at about 16 weeks.

How is it performed?

A blood sample is taken and is then measured for the levels of at least three substances (a protein called alpha-fetoprotein and two hormones) in your blood.

What if the results are abnormal?

Nine of ten women receive normal results and breathe a sigh of relief. But if you're one of the 10 percent with abnormal results, don't panic -- chances are the baby is fine. Because this is a screening test, many more babies are shown to be at risk than actually have problems. In fact, for every 100 women with abnormal results, only 2 or 3 have a baby with birth defects. In most cases, abnormal test results occur because your baby is either a few weeks older or younger than originally thought, which throws off the reading. So if you have an abnormal result, the first step your healthcare provider will take is to give you an ultrasound to recalculate your baby's age. The ultrasound will also show if you're carrying twins -- another common cause of an abnormal blood test. If your baby's age or the fact that you're carrying multiples explains your result, you probably won't need any further tests.

What does it test for?

If your blood test was abnormal and the ultrasound didn't uncover the cause, your doctor may recommend an amniocentesis to test for birth defects. Also, because the risk of having a baby with birth defects rises as you age, your healthcare provider may want to do an amniocentesis as a precautionary measure between 15 and 18 weeks if you're over 35. Amniocentesis is accurate in diagnosing chromosomal birth defects (such as Down syndrome), a wide variety of genetic birth defects (such as muscular dystrophy and cystic fibrosis), and certain malformations (such as neural tube defects or spina bifida).

How is it performed?

One or two tablespoons of amniotic fluid are extracted from your abdomen with a thin needle. Some women say amnio doesn't hurt at all; others feel cramping when the needle enters the uterus or pressure during the short time the fluid is being withdrawn. One to 2 percent of women experience cramping, leaking amniotic fluid, or spotting after the procedure (which you should report to your doctor). Most physicians recommend that you rest for several hours after the test.

Is it dangerous?

Amniocentesis does pose a small risk of miscarriage, so it's your choice whether or not to go ahead with the procedure. According to the Centers for Disease Control and Prevention (CDC), the rate of miscarriage after amniocentesis is between one in 200 and one in 400. The risk is three times higher if it's performed in the first trimester. It also carries an extremely low risk of uterine infection (less than one in 1,000), which may lead to miscarriage.

What if the results are abnormal?

The results can take two to three weeks, which is a long time to wait if you're nervous. However, the test provides you an opportunity to prepare ahead of time if there are birth defects -- and in some cases, problems can be treated while the baby is still in the womb.

What does it test for?

CVS tests for the same birth defects as amniocentesis, with the exception of neural tube defects. Both amniocentesis and CVS are more than 99 percent accurate in ruling out chromosomal birth defects and specific genetic problems. The benefit of CVS is that the results come back much more quickly than with an amnio. The drawback is that it's slightly more likely to give inconclusive results, which means you may need to get an amnio as well. If you already have a child with a birth defect, had a previous pregnancy affected by a birth defect, or have family members with these disorders and are especially nervous about them, CVS may be something to consider.

When is it done?

It's usually performed between 10 and 12 weeks.

How is it performed?

In CVS, a physician takes a small piece of the chorionic villi, which are wisps of tissue attached to the uterine wall. The cells are collected either by transcervical CVS, in which a thin tube is inserted through your vagina and cervix, or by transabdominal CVS, in which a needle is inserted through your abdomen. The villi have the same biochemical makeup as the fetus and can show the majority of potential birth defects.

As with amniocentesis, some women say that CVS is painless, but others experience cramping when the sample is taken. Rest after the procedure; 1 in 3 women have some bleeding or spotting, which usually stops within a few days. You should always report these symptoms to your healthcare provider. Test results take one to two weeks.

Is it dangerous?

According to the CDC, between one in 100 and one in 200 women miscarry after CVS. So you may want to consider all your options before deciding to have a CVS performed.

The wait for test outcomes can seem endless, but you'll probably get good news. More than 95 percent of women who have amniocentesis or CVS find out that the results don't show the disorders for which their baby was tested. A small number of couples learn that their baby does have a birth defect. While this news can be devastating, prenatal diagnosis can sometimes improve the outlook for your baby. A few disorders can be treated before birth with drug therapy, blood transfusions, or even surgery. If the baby has a condition that can't be treated prenatally, the early diagnosis permits parents to prepare emotionally and to plan with their physician the safest timing, location, and method of delivery. This can make a difference: Studies suggest that babies with spina bifida, for instance, suffer less paralysis when delivered by c-section before labor has a chance to begin.

All of these issues illustrate why it's important for pregnant women to get early prenatal care. See a doctor as soon as you know you're pregnant or even before conception to help ensure that you and your baby get the best start.

All content here, including advice from doctors and other health professionals, should be considered as opinion only. Always seek the direct advice of your own doctor in connection with any questions or issues you may have regarding your own health or the health of others.