Maternal serum screening and routine 18-week ultrasound in the detection of all chromosomal abnormalities

Over a 15-month period, maternal serum screening (alpha fetoprotein, oestriol, chorionic gonadotrophin) and ultrasound were evaluated in the detection of all chromosomal abnormalities. Of the 981 screened, there were 8 chromosomally abnormal pregnancies. Six of these were considered to be at increased risk on serum screening, all of which were detected. Of the remaining 2, one was detected by ultrasound and the other resulted in a liveborn baby with trisomy 21. The positive and negative predictive values for serum screening for all chromosomal abnormalities was 7.8% and 99.9% respectively. The sensitivity and specificity was 87.5% and 91.5% respectively. Serum screening is useful in the detection of many chromosomal abnormalities, not just Down syndrome. The combination of maternal serum screening and ultrasound has a high negative predictive value and is valuable in providing reassurance of no underlying chromosomal abnormality. With a positive predictive value of 7.8% a chromosomal abnormality will be found once in every 13 amniocenteses performed.