Second Trimester Prenatal Screening for Down Syndrome in Thai Pregnant Women Using Alpha-fetoprotein and Free Beta-hCG

Abstract

Objective To study the values of second trimester prenatal screening for Down syndrome using
alpha-fetoprotein and free beta-hCG in Thai pregnant women.
Design A prospective descriptive study (diagnostic test).
Setting Department of Obstetrics and Gynecology, Charoenkrung Pracharuk Hospital.
Methods Alpha-fetoprotein (AFP) and free beta-hCG (free b-hCG) were measured in serum
samples from 1,808 singleton Thai pregnant women at 15-20 weeks of gestation. Gestational
ages were determined by ultrasonographic parameters measured at the same visit as the
test for all cases. The gestational-age-specific and weight-adjusted medians for serum AFP
and free b-hCG were calculated. The risk for fetal Down syndrome (DS) was derived by
mathematical modeling of the medians together with maternal age. Amniocentesis were
offered to women with a calculated DS risk of 1:270 or greater and to women aged 35 years or
over.
Result A total of 141 women (7.8%) were classified as being high risk for DS, including 4.1%
(53/1,289) of those younger than 35 years of age and 17.1% (89/519) of those who aged 35
years or over. Five out of six cases of Down syndrome were screen-positive, corresponding to
a detection rate of DS of 83% with a 7.5% false positive rate. A case of trisomy13, 1 case of
Klinefelter syndrome and 1 case of balanced translocations were found screen-positive for DS.
A case of trisomy18 affected by an omphalocele was screened negative for DS with high level
of AFP (6.35 MoM) and low level of free b-hCG (0.44 MoM). The additional value of prenatal
screening was a detection rate for neural tube defect (NTD) of 80% with a 2.8% false positive
rate.
Conclusion Maternal serum screening using double biochemical markers (AFP and free b-hCG)
in combination with gestational dating by ultrasonography is effective in the detection of fetal
Down syndrome and possibly other abnormalities in Thai pregnant women.