Prenatal Screening

According to the March of Dimes, about 120,000 babies are born with birth defects each year. A birth defect is an abnormality of structure, function, or metabolism present at birth that results in physical or mental disabilities or possibly death.

Testing for birth defects may be nerve-wracking for expectant parents, but it can also bring peace of mind.

Down Syndrome

One of the most frequent prenatal tests doctors perform is for the detection of Down syndrome.

In Down syndrome, the presence of an additional chromosome causes a combination of birth defects, such as mental retardation, characteristic facial features, heart defects, increased infections, trouble with vision and hearing, and other health problems.

Down syndrome is one of the most common genetic birth defects, affecting approximately one in 800 babies.

Prenatal Tests

The American College of Obstetricians and Gynecologists (ACOG), recommends that pregnant women of all ages consider screening options before the 15th week of pregnancy to assess their risk of having a baby with Down syndrome.

In the first trimester, the organization recommends using a painless, non-invasive screening test called nuchal translucency (NT), followed by a blood test.

The NT screening uses ultrasound on the abdominal area of the woman and measures the skin thickness on the back of a fetus's neck. The blood test that follows measures levels of substances called pregnancy-associated plasma protein A and human chorionic gonadotropin (hCG).

In the second trimester, sometime between 15-20 weeks of pregnancy, your doctor may give you a screen called a multiple marker test. This test measures the level of several substances in the blood, including alpha-fetoprotein and the hormones hCG, estriol, and inhibin-A.

Results from first and second trimester tests can identify 85-96 percent of fetuses likely to be born with Down syndrome, as well as the likelihood of other conditions such as trisomy 18 and neural tube defects. The screening can pinpoint these genetic disorders as early as 11 weeks of gestation.

"It is so beneficial because it gives women more information earlier, allowing more time to consider the effects," says Jeff Traynor, MD, perinatologist at John Muir Health.

Diagnosis

You should consider a number of things when deciding which screening test is best, including:

• How far along the pregnancy is
• Whether you are carrying multiple fetuses
• Personal and family history
• Availability of tests

Although the NT test is a helpful indicator of key genetic birth disorders, it doesn't directly test for chromosomal problems — it just gives expecting parents a better idea of their baby's statistical likelihood of having a problem.

With the help of your practitioner or a genetic counselor, you'll then want to decide whether the results indicate a high enough risk that you want to have more testing to get a definitive diagnosis — that is, to see whether your baby really does have a chromosomal defect.

Tests that can diagnose a chromosomal defect include chorionic villus sampling (CVS) and amniocentesis. "Prenatal testing can be emotional and understandably stressful for many parents," says Dr. Traynor.

"But once the testing is completed, the knowledge that is gained can help parent's better plan for their individual situation."