Maternal serum screening for Down's syndrome: the effect of routine ultrasound scan determination of gestational age and adjustment for maternal weight.

OBJECTIVE

To investigate the effect of using a routine ultrasound estimate of gestational age and maternal weight adjustment on maternal serum alpha-fetoprotein (AFP), unconjugated oestriol (uE3) and human chorionic gonadotrophin (hCG) levels in antenatal screening for Down's syndrome.

DESIGN

Women with a singleton pregnancy without Down's syndrome were screened using the three serum markers and an estimate of gestational age based on 'dates' (time since first day of the last menstrual period) and one based on an ultrasound scan examination was recorded together with maternal weight.

SETTING

Women attending the Homerton Hospital, Hackney, for their antenatal care between February 1989 and January 1990.

SUBJECTS

2113 women with a singleton pregnancy without Down's syndrome.

RESULTS

The use of ultrasound to estimate gestational age (usually based on the biparietal diameter of the fetal skull) led to a significant reduction in the variance of each marker at a given week of pregnancy. The level of each marker was negatively associated with maternal weight, so that adjustment for weight also led to a reduction in variance. These data on gestational age and maternal weight, taken together with published data on pregnancies associated with Down's syndrome, indicate that the routine use of ultrasound to estimate gestational age will increase the detection rate from 58% to 67% while maintaining the false-positive rate at 5%, or reduce the false-positive rate from 5.7% to 3.1% while maintaining the detection rate at 60%. Routine maternal weight adjustment for the serum marker levels was much less useful, increasing the detection rate by about 0.5% for a given false-positive rate, or reducing the false-positive rate about 0.1% for a given detection rate.

CONCLUSION

An ultrasound gestational age estimate available at the time of Down's syndrome screening confers a substantial advantage to screening performance with a further small benefit resulting from maternal weight adjustment, which is worth adopting if it can be done without difficulty or extra cost.