What to expect when you're expecting (26 page)

Have tested positive on a prenatal screening test.

Why go through diagnostic tests if there’s some risk involved? The best reason for prenatal diagnosis is the reassurance it almost always brings. The vast majority of babies whose possibly- at-risk moms undergo such testing will receive a perfect bill of health—which means that mom and dad can quit worrying and start enjoying their pregnancy.

What is it?
One of the simplest screening tests is the ultrasound. Using sound waves so high they can’t be heard by the human ear, sonography allows visualization and “examination” of the fetus without X-rays. Though sonography tends to be fairly accurate for most uses, in screening for birth defects the test can yield some false negatives (it seems as though everything is fine, but it’s not) and some false positives (it looks as though there is a problem, but there really isn’t).

A first-trimester ultrasound (usually a basic level 1 ultrasound) is performed in order to:

Confirm the viability of a pregnancy

Date a pregnancy

Determine the number of fetuses

Determine the cause of bleeding, if any

Locate an IUD that was in place at the time of conception

Locate the fetus prior to CVS or amniocentesis

Assess the risk for chromosomal abnormality as part of a screening test

How is it done?
Though ultrasound examination is often performed using a wand or transducer over the abdomen (transabdominal), during the first trimester a transvaginal exam may be preferred, especially early on. The procedures can last from 5 to 30 minutes and are painless, except for the discomfort of the full bladder necessary for the first-trimester transabdominal exam.

During either exam, you’ll lie on your back. For the transabdominal, your bare abdomen is spread with a film of gel that will improve the conduction of sound. A transducer is then moved slowly over your belly. For the transvaginal, the transducer is inserted into your vagina. In both procedures, the instruments record echoes of sound waves as they bounce off parts of your baby and translate them into pictures on a viewing screen.

When is it done?
Ultrasound is done anytime during the first trimester of pregnancy, depending on the reason for performing one. A gestational sac can be visualized on ultrasound as early as 4½ weeks after your last period; a heartbeat can be detected as early as. to 6 weeks (though it might not be detected that early in all cases). For information on second-trimester ultrasound, see
page 66
.

How safe is it?
After many years of clinical use and study, no known risks and a great many benefits have been associated with the use of ultrasound. Many practitioners order ultrasound exams routinely, at least once in a woman’s pregnancy. Still, it’s generally recommended by most experts that ultrasound be used in pregnancy only when a valid indication exists.

What is it?
First-trimester combined screening involves both an ultrasound and a blood test. First the ultrasound measures a thin layer of fluid that accumulates in the back of the baby’s neck, called nuchal translucency (NT). Increased fluid
may
indicate an increased risk of chromosomal abnormalities such as Down syndrome, congenital heart defects, and other genetic disorders.

Then the blood test measures for high levels of PAPP-A (pregnancy- associated plasma protein A) and hCG, two hormones produced by the fetus and passed into the mother’s bloodstream. These levels, combined with the NT measurement and the mother’s age, can provide a risk assessment for Down syndrome and trisomy 18.

A few medical centers also look for the absence of a nasal bone in the fetus during the ultrasound. Some studies have shown that the lack of a nasal bone during the first trimester may indicate a higher risk of Down syndrome, but other studies haven’t backed that up, making this type of assessment somewhat controversial.

Though the first-trimester combined screening test can’t give you the definite diagnosis you’d get from more invasive diagnostic tests, it can help you decide whether you want to undergo
diagnostic testing. If your screening test shows that your baby may be at an increased risk of having a chromosomal defect, a diagnostic test such as chorionic villus sampling (CVS; see this page) or amniocentesis (see
page 64
) will be offered. If the screening test doesn’t show an increased risk, your practitioner might recommend that you take the quad screen test in the second trimester (see
page 63
) to rule out neural tube defects. And because increased NT measurements are also associated with fetal heart defects, your practitioner might suggest a fetal echocardiogram at around 20 weeks to screen for heart defects if your levels are high. Increased NT measurements may also be linked to a very slightly higher risk of preterm labor, so you may be monitored for that as well.

When is it done?
First-trimester combined screening is performed between 11 and 14 weeks of pregnancy.

How accurate is it?
This screening test doesn’t directly test for chromosomal problems, nor does it diagnose a specific condition. Rather, the results merely provide you with your baby’s statistical likelihood of having a problem. An abnormal result on the combined screening test doesn’t mean that your baby has a chromosomal problem, just that he or she has an increased risk of having one. In fact, most women who have an abnormal result on their screening test go on to have a perfectly normal and healthy child. At the same time, a normal result is not a guarantee that your baby is normal, but it does mean that it is very unlikely that your baby has a chromosomal defect.

The first-trimester combined screening can detect approximately 80 percent of Down syndrome and 80 percent of trisomy 18 problems.

How safe is it?
Both the ultrasound and the blood test are painless (unless you count the needle prick for the blood test) and carry no risk to you or your baby. But there is one caveat. This type of screening test requires sophisticated ultrasound technology. To assure the best possible accuracy of the results, it should only be done with special equipment (a high-quality ultrasound machine) and by physicians and sonographers with special ongoing training. Keep in mind, too, that a risk of the test is that a false-positive (screen-positive) result may lead to follow-up procedures that present greater risk. Before you consider taking any action on the basis of prenatal screening, be sure an experienced physician or genetic counselor has evaluated the results. Get a second opinion if you have any doubts.