First trimester Down syndrome screening by nuchal translucency in Thai population

Abstract

Purpose: To examine the significance of the fetal nuchal translucency (NT) at 10–13 + 6 weeks' gestation in the prediction of DS in a Thai population.Methods: Total 2353 pregnant women were recruited from January 1996 to June 1999. Almost all women underwent transabdominal scanning (TAS) (Toshiba 140, TAS probe 5.0 MHz Toshiba PVF‐5.00 MT). When the result showed an increased risk for DS (>2.5 mm NT), the pregnant woman was counseled and the invasive test was optional. All the newborns were physically examined thoroughly by pediatricians to detect the markers for major chromosomal anomalies such as DS, trisomy 18 and 13. For the suspected DS or other chromosomal abnormalities, the newborn's blood was sent for the chromosome study.Results: The mean maternal age was 28.71 + 0.13 years and 2037 pregnant women (86.57%) were <35 years old. The mean gestational age was 11.94 + 1.07 weeks. Sixty‐five pregnant women who had NT > 2.5 mm decided to have the invasive tests. Sixty had normal karyo‐types while five had abnormal chromosome abnormalities (one trisomy 13, two trisomy 18 and two trisomy 21). All of them were >35 years old. Sixty pregnant women with normal chromosome fetuses had detailed ultrasound scanning at 18–20 weeks' gestation and screening for maternal toxoplasmosis, cytomegalovirus, rubella virus, and herpes virus. The infection screen was negative in all cases. There were 20 congenital heart diseases, four fetal deaths in utero at 28–30 weeks, one Bart's hydrops fetalis and 10 abortion. The rest had normal pregnancy outcomes and NT resolved by 20 weeks. There were 316 (13.43%) cases with maternal age >35 years. Twenty‐three had >2.5 mm. NT and all of them had the invasive tests which revealed five abnormal chromosomes (as mentioned above). However, no DS fetuses were detected in <35 years pregnant women. Two hundred pregnant women with normal NT decided to have the invasive tests due to advanced maternal age and all of them had normal karyo‐types. The sensitivity, specificity, positive predictive values and negative predictive values for trisomy 21, 18 or 13 with different cut‐off values for fetal NT were studied. This is the first report of the first trimester NT screening for DS in Thai people. The cut‐off of >2.5 mm NT was used to increase sensitivity. We can detect all the cases of DS with this cut‐off value but one case of DS will be missed if we use the cut‐off of >3.0 mm.Conclusions: This study revealed the feasibility in NT measurement for DS screening in the first trimester of pregnancy in Thai population. This test can increase detection rate of DS in the high‐risk pregnant women, who did not accept the invasive prenatal diagnosis procedures, as well as in the low risk pregnancy.