First-Trimester Screening

First trimester screening is a relatively new prenatal test that offers early information about a baby’s health. First trimester screening is performed between weeks 11 and 14 of pregnancy which is earlier than many other prenatal tests. This screening process can help to determine the risk of the fetus having certain birth defects such as Down syndrome and other chromosomal abnormalities as well as major congenital heart problems.

First trimester screening is a combination of a blood test to measure levels of two pregnancy-specific substances-- pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (hCG) --- and a specialized ultrasound exam called Nuchal Translucency (NT) which measures the size of a specific area at the back of your baby's neck called a nuchal fold. Babies with abnormalities tend to accumulate more fluid at the back of their neck during the first trimester, causing this clear space to be larger than average.

You don't need to do anything special to prepare for first trimester screening. You can eat and drink normally before both the blood test and the ultrasound exam.

Using your age and the results of the blood test and the ultrasound, your health care provider can gauge your risk of carrying a baby who has Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18). Down syndrome causes varying degrees of mental retardation with some physical anomalies possible. Edwards syndrome causes more severe retardation and possible physical anomalies that may be life threatening.

ACOG (American College of Obstetrics and Gynecology) states “first-trimester screening using both nuchal translucency measurement and biochemical markers is an effective screening test for Down syndrome in the general population. …this screening strategy results in a higher Down syndrome detection rate than does the second-trimester maternal serum triple screen…”. They also state “screening and invasive diagnostic testing for aneuploidy should be available to all women who present for prenatal care before 20 weeks of gestation regardless of maternal age.”

Like other screening tests, a first trimester screening won't give you a diagnosis but if your risk level is low, first trimester screening can offer reassurance of a healthy pregnancy. If your risk level is moderate or high, you may choose to follow first trimester screening with more invasive diagnostic testing such as chorionic villus sampling (CVS) or amniocentesis to determine whether the baby actually has Down syndrome or Edwards syndrome.