n addition, maternal blood is measured for levels of a protein called pregnancy-associated
plasma protein A (PAPP-A) and a hormone known as human chorionic gonadotropin (HCG).
In fetuses with Down syndrome, the nuchal translucency measurement is increased,
HCG is increased and PAPP-A is decreased .
How Accurate Is First Trimester Screening?
First trimester screening can
detect up to 85% of babies with Down syndrome and trisomy 18. Up to 15% of Down
syndrome and trisomy 18 cases will not be detected. The
test incorrectly reports an increased risk for Down syndrome (a false positive
result) approximately 5 % of the time [2-4] .
What Do the Results Mean?
The first trimester screen is not diagnostic. The results are an estimate of the risk for
Down syndrome or trisomy 18 in a particular fetus.
A positive test result indicates an increased risk for Down syndrome above that of a 35 year old at the
same gestational age (1/240-1/260).A positive result for trisomy 18 indicates
a risk of 1/150 (0.67%) or greater.
If the results are positive the genetic counselor will explain the results, and discuss options of
further testing such as chorionic villus sampling.
If the results are
negative, then maternal serum AFP screening for neural tube defects at 16 weeks
and a high-resolution ultrasound at 18 to 20 weeks are recommended.
Women over age 34 or
those with additional risk factors may still choose to have chorionic villus
sampling or amniocentesis to test for other abnormalities.
Health Insurance Coverage
The cost of screening includes
genetic counseling, ultrasound, and a laboratory fees.
Patients wishing to have first trimester screening should call their insurance
carriers to verify coverage for the first trimester screen. The procedures are covered by
most insurance carriers. Patients also have the option to self pay for first
1.Canick JA, Kellner LH. First trimester serum
screening for aneuploidy: Serum biochemical markers. Semin Perinatol
2. Wapner R, First-trimester screening for
trisomies 21 and 18.N Engl J Med. 2003 Oct 9;349(15):1405-13. PMID: 14534333
3. Malone FD, D'Alton ME; Society for Maternal-Fetal Medicine. First-trimester sonographic screening for
Down syndrome. Obstet Gynecol. 2003;102(5 Pt 1):1066-1079.PMID: 14672489
4. Wald NJ, Rodeck C, Hackshaw AK, et al. First and second trimester antenatal screening for Down's
syndrome: The results of the Serum, Urine and Ultrasound Screening Study (SURUSS).
Health Technol Assess. 2003;7(11):1-77. PMID: 12709291