To compare the effectiveness of different screening policies for the antenatal detection of Down's syndrome.
Retrospective six year survey.
Maternity units of eight districts.
Women who completed their pregnancies between 1 January 1994 and 31 December 1999 (155 501 deliveries).
Main outcome measures
Cases of Down's syndrome identified before 24 weeks' gestation.
335 cases of Down's syndrome were identified, 323 in continuing pregnancies or liveborn children. Of these, 171 were identified antenatally. Seven different screening policies were used, in three principal groups: serum screening offered to all mothers, maternal age with serum screening or nuchal translucency available to limited groups, and maternal age combined with anomaly scans. The districts that used serum screening detected 57%, those using maternal age plus serum or nuchal translucency screening 52%, and those using a maternal age of ⩾35 and anomaly scans detected 54%. The least successful district, which offered amniocentesis only to women aged over 37 years, detected only 31%. If amniocentesis had been offered from 35 years, as in all other districts, the detection rate would have risen to 54%. Across the region 15% (range 12-20%) of pregnant women were 35 years or more at delivery, and 58% (33-69%) of infants with Down's syndrome were born to women in this age range.
Current additional serum or nuchal translucency screening techniques for antenatal detection of Down's syndrome are less advantageous than previously supposed. More pregnant women were aged over 35 than has been presumed in statistical models used in demonstration projects of serum screening and, as a result, the proportion of affected fetuses in this age group is much greater than predicted.
What is already known on this topicSerum screening for Down's syndrome has been presumed to be more effective than screening by maternal ageThere have been no controlled studies comparing serum screening with screening by maternal age, and its greater efficacy has been presumed from mathematical modelling, which assumed that only 5% of pregnant women were aged over 35 yearsThe modelling predicted that only 20-30% of cases of Down's syndrome would arise in women aged over 35 and made no allowance for the effects of routine anomaly scanningWhat this study adds15% of pregnant women were aged over 35 years, more than double the 5-7% presumed in statistical models of screening58% of babies with Down's syndrome were born to women aged 35 years or moreSerum screening and nuchal scanning did not achieve significantly higher antenatal detection rates of Down's syndrome than the use of maternal age and routine anomaly scanning