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Before chorionic villus sampling at 10–13 weeks' gestation, 453 women had the crown–rump length and nuchal translucency (NT) measured with transabdominal ultrasound. There were 19 aneuploid pregnancies (ten cases of trisomy 21, six of trisomy 18, one of 47+marker, one 47,%, and one 45,X mosaic). Average NT was 1·7 mm (range 0–5 mm), correlating with the crown–rump length, but not maternal age. A static cut‐off of 2·5 mm gave a false‐positive rate of 1·3 per cent for crown–rump length between 30 and 35 mm, rising to 13 per cent in fetuses with a crown–rump length between 50 and 65 mm. This gave an overall false‐positive rate of 5·5 per cent for a detection rate of 30 per cent for trisomy 21. Applying a dynamic action limit (95th centile), the false‐positive rate remained at 5 per cent irrespective of the crown–rump length, detecting 30 per cent of trisomy 21 and 36·8 per cent of all aneuploidies. Raising the action limit to the 97·5th centile halved the false‐positive rate (2·5 per cent), with no change in trisomy 21 detection and only a slight decrease in aneuploidy detection (31·6 per cent). Aneuploid fetuses showed normal first‐trimester growth. NT increases with gestational age, making a dynamic action limit necessary to decrease the false‐positive rate, while maintaining aneuploidy detection rates. Aneuploidy does not cause significant first‐trimester growth retardation, enabling normal ranges for NT with crown–rump length to apply.
Before chorionic villus sampling at 10–13 weeks' gestation, 453 women had the crown–rump length and nuchal translucency (NT) measured with transabdominal ultrasound. There were 19 aneuploid pregnancies (ten cases of trisomy 21, six of trisomy 18, one of 47+marker, one 47,%, and one 45,X mosaic). Average NT was 1·7 mm (range 0–5 mm), correlating with the crown–rump length, but not maternal age. A static cut‐off of 2·5 mm gave a false‐positive rate of 1·3 per cent for crown–rump length between 30 and 35 mm, rising to 13 per cent in fetuses with a crown–rump length between 50 and 65 mm. This gave an overall false‐positive rate of 5·5 per cent for a detection rate of 30 per cent for trisomy 21. Applying a dynamic action limit (95th centile), the false‐positive rate remained at 5 per cent irrespective of the crown–rump length, detecting 30 per cent of trisomy 21 and 36·8 per cent of all aneuploidies. Raising the action limit to the 97·5th centile halved the false‐positive rate (2·5 per cent), with no change in trisomy 21 detection and only a slight decrease in aneuploidy detection (31·6 per cent). Aneuploid fetuses showed normal first‐trimester growth. NT increases with gestational age, making a dynamic action limit necessary to decrease the false‐positive rate, while maintaining aneuploidy detection rates. Aneuploidy does not cause significant first‐trimester growth retardation, enabling normal ranges for NT with crown–rump length to apply.
We assayed alpha fetoprotein (AFP) in serum samples from 2,735 women during 14 to 20 weeks of gestation. Serum AFP levels were elevated in the presence of neural tube defect (NTD) and gut atresia in fetus, twin pregnancies, preterm delivery and neonatal complications. In two of the 23 cases of fetal NTD the diagnosis was suggested by AFP assays, with apparently normal ultrasound findings. Low maternal serum AFP levels were associated with chromosomal abnormalities and hydatidiform molar pregnancy. Calculation of risk for Down syndrome based on maternal serum AFP and maternal age helped to reduce the number of women requiring amniocentesis. Maternal serum AFP assay was helpful in the management of threatened abortion, suspected intrauterine death, and maternal toxoplasma infection. In seven cases where maternal serum AFP was high but ultrasound studies were normal, male babies were delivered. Thus maternal serum AFP assay proved useful in narrowing down the group of women requiring more detailed surveillance and diagnostic studies.
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.
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