Prenatal Tests and Procedures, Nuchal Transluncency Screening
Posted Dec 02 2008 9:51am
While you are pregnant your doctor or midwife may suggest a number of laboratory tests, ultrasound exams, or other screening tests. Read on to find out the basics of the most common tests done during pregnancy.
Screening tests measure the risk of having a baby with some genetic birth defects. Birth defects are caused by problems with a baby's genes, inherited factors passed down from the mother and the father. Birth defects can also occur randomly in people with no family history of that disorder. Women over the age of 35 have the greatest risk of having babies with birth defects.
The benefit of screening tests is that they do not pose any risk to the fetus or mother. But screening tests cannot tell for sure if the baby has a birth defect. So, they do not give a "yes" or "no" answer. Instead, screening tests give the odds of your baby having a birth defect based on your age. Women under the age of 35 will find out if their risk is as high as that of a 35 year old woman. For women over age 35, screening tests will help them find out if their risk for their age is higher or lower than average.
Some common screening tests used during pregnancy include:
The best time to receive this test is between 18 and 20 weeks of pregnancy. Most major problems with the way your baby might be formed can be seen at this time. But some problems like clubbed feet and heart defects can be missed on ultrasound. Your doctor also will be able to see if your baby has any neural tube defects, such as spina bifida. This test is not the most accurate for finding out whether your baby has Down syndrome. Only 1 in 3 babies with Down syndrome have an abnormal 2nd trimester ultrasound. In most cases, your doctor can find out the sex of your baby by using ultrasound. Maternal serum marker screening test
This blood test can be called by many different names including multiple marker screening test, triple test, quad screen, and others. This test is usually given between 15 and 20 weeks of pregnancy. It checks for birth defects such as Down syndrome, trisomy 18, or open neural tube defects. Doctors take a sample of your blood. They check the blood for 3 chemicals: alpha-fetoprotein (AFP) (made by the liver of the fetus), and two pregnancy hormones: estriol and human chorionic gonadotropin (hCG). Sometimes, doctors test for a fourth substance in the blood called inhibin-A. Testing for inhibin-A may improve the ability to detect fetuses with a high risk of Down syndrome.
Higher levels of AFP are linked with open neural tube defects. In women age 35 and over, this test finds about 80% of fetuses with Down syndrome, trisomy 18, or an open neural tube defect. In this age group, there is a false positive rate (having a positive result without actually having a fetus with one of these health problems) of 22%. In women under age 35, this test finds about 65% of fetuses with Down syndrome, and there is a false positive rate of about 5%.
Nuchal translucency screening (NTS)
This new type of screening can be done between 11 and 14 weeks of pregnancy. It uses an ultrasound and blood test to calculate the risk of some birth defects. Doctors use the ultrasound exam to check the thickness of the back of the fetus' neck. They also test your blood for levels of a protein called pregnancy-associated plasma protein and a hormone called human chorionic gonadotropin (hCG). Doctors use this information to tell if the fetus has a normal or greater than normal chance of having some birth defects.
In an important recent study, NTS found 87% of cases of Down syndrome when done at 11 weeks of pregnancy. When NTS was followed by another blood test done in the second trimester ( maternal serum screening test), 95% of fetuses with Down syndrome were identified.
Like all screening tests, the results are sometimes misleading. In 5% of women who have NTS, results show that their babies have a high risk of having a birth defect when they are actually healthy. This is called a false positive. To find out for sure if the fetus has a birth defect, NTS must be followed by a diagnostic test like chorionic villus sampling or amniocentesis .
NTS is not yet widely used. If you are interested in NTS, talk to your doctor. If she is unable to do the test, she can refer you to someone who can. You should also call your insurance company to find out if they cover the cost of this procedure. NTS allows women to find out early if there are potential health problems with the fetus. This may help them decide whether to have follow-up tests.
Diagnostic tests can give definite "yes" or "no" answers about whether your baby has a birth defect. But, unlike screening tests, they are invasive or come with a risk of miscarriage. Amniocentesis and chorionic villus sampling (CVS) are the two most commonly used. Both tests are more than 99% accurate for finding these problems. These tests also can tell you your baby's sex. In most cases, results take about two weeks. Amniocentesis
This test is performed in pregnancies of at least 16 weeks. It involves your doctor inserting a thin needle through your abdomen, into your uterus, and into the amniotic sac to take out a small amount of amniotic fluid for testing. The cells from the fluid are grown in a lab to look for problems with chromosomes. The fluid also can be tested for AFP. About 1 in 200 women have a miscarriage as a result of this test. Chorionic villus sampling (CVS)
This test is performed between 10 and 12 weeks of pregnancy. The doctor inserts a needle through your abdomen or inserts a catheter through your cervix in order to reach the placenta. Your doctor then takes a sample of cells from the placenta. These cells are used in a lab to look for problems with chromosomes. This test cannot find out whether your baby has open neural tube defects. About 1 in 200 women have a miscarriage as a result of this test.
Source: National Women’s Health Information Center, U.S. Department of Health and Human Services, www.womenshealth.gov