Since the 1980’s, pregnant women have been offered second-trimester testing for Down Syndrome and other common genetic defects. This has involved doing testing at about 15-16 weeks of pregnancy to measure levels of alpha-fetoprotein (AFP) and other substances in the mother's blood.

More recently, first-trimester screening for Down Syndrome and other chromosomal abnormalities has been developed. Between 11 and 14 weeks, an ultrasound examination is done to measure the thickness of an area at the back of the fetal neck (nuchal translucency, or NT), combined with measuring levels of two substances in the mother’s blood (free beta subunit of human chorionic gonadotropin, or HCG, and pregnancy-associated plasma protein-A, or PAPP-A). Results are available in 6-7 days.

Like the AFP testing, this is a screening test: the results indicate that the woman is at increased risk, and a definitive diagnostic test (amniocentesis or CVS) is used to determine if the fetus is affected.The ultrasound procedure used to measure NT requires special training, skills, and ongoing quality assurance, and we are very pleased that our sonographers are now trained and certified to do NT scans. It is not painful or risky to either mother or baby.

First-trimester testing, when results are negative, can provide earlier reassurance to concerned parents than second-trimester screening. If results are positive, definitive testing can be done to either allow patients to have more time to prepare for a child with health problems, or to have a safer and more private procedure if a pregnancy termination is elected.

First-trimester testing is estimated to identify 91% of fetuses with Down Syndrome. It can also identify 40% of fetal heart defects, 97% of cases of Trisomy 18, and some other birth defects. It is not however useful as a screening test for spina bifida, which can usually be detected by a combination of measuring AFP at 15+ weeks and an ultrasound at 18-20 weeks. We offer all our patients an 18-20 week ultrasound for examination of fetal anatomy.

Further information on first-trimester screening is available at the website of the American College of Obstetricians and Gynecologists: http://www.acog.org/publications/patient_education/bp165.cfm