Nuchal translucency as a method of first-trimester screening for aneuploidy

To determine the effectiveness of nuchal translucency (NT) screening in predicting aneuploidy and structural abnormalities in a South African population.

Descriptive study. Setting. Chris Hani Baragwanath Hospital fetal medicine unit.

An adjusted risk was derived from the combination of maternal age-related risk and the risk derived from NT screening. A positive screen was denoted by an adjusted risk of more than 1/300 and a negative screen by an adjusted risk of less than 1/300. In order to determine the number of undiagnosed abnormalities in the group, all babies were examined by a paediatrician at birth to detect and describe dysmorphic features.

A total of 428 patients underwent first-trimester screening between July 2003 and July 2005. Three per cent were lost to follow-up. Of the 415 patients analysed, 59 screened positive and 356 screened negative. The mean age for both groups of patients was 30.1 years. Of the 57 patients who screened positive, 24 elected to have chorionic villus sampling (CVS). This resulted in the detection of 6 chromosomal abnormalities and 2 structural abnormalities. Among the remaining 356 patients, who had screened negative, 2 had an increase in the adjusted risk when the risk was compared with the background risk, and 1 chromosomal abnormality was detected in this group; 8 elected to have CVS because of a previous history of a chromosomal abnormality, and there were no abnormalities among them.

The use of these screening methods has enabled prenatal karyotyping to become cost effective, and allows concentration on pregnancies at highest risk for chromosomal abnormalities, regardless of age.