Introduction

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Screening for Down's syndrome has become an accepted part of routine antenatal care, but there is wide variation between districts in the policy used. On the advice of the antenatal subgroup of the National Screening Committee in April 2001 the UK government announced that by 2004 all pregnant women should be offered serum screening in the second trimester to increase the antenatal detection of Down's syndrome and to reduce the amniocentesis rate.

There have been no controlled trials showing the effectiveness of this system compared with screening by maternal age in units that offer routine anomaly scanning. We carried out a comparative audit of antenatal screening in adjacent health districts to determine whether serum screening is justified by an increase in the detection rate of Down's syndrome or by a reduction in the rate of invasive procedures.

	Methods

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We used the Wessex antenatally detected anomalies register to ascertain all cases of Down's syndrome detected in pregnancy (including deliveries, miscarriages, or terminations) or postnatally in the region in the six years from 1 January 1994 to 31 December 1999. We considered cases to have been successfully diagnosed antenatally if they were detected before 24 weeks' gestation, a stage in pregnancy when termination can still be offered.

	Results

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In the six years studied, 155 501 babies were delivered in the region in the eight district hospitals, their associated community hospitals, or at home. In total 335 cases of Down's syndrome were detected during pregnancy or in newborn babies, giving an overall incidence of 2.1 per 1000 deliveries (95% confidence interval 1.9 to 2.3). In 1989 the national incidence was 1.4 per 1000 live births.¹ In 12 cases the pregnancies had already failed as a result of missed abortions or miscarriage and so would not have led to a live child. We confined the analysis to the 323 continuing pregnancies.

Across the region 15% of pregnant women were aged 35 or more. Overall, 186 (58%, 53% to 63%) affected pregnancies were in women aged 35 years and over, suggesting that if maternal age was the only indication for offering invasive testing a high proportion of cases would be detected. Among the eight districts, there were seven different screening policies for Down's syndrome (table 1).

Detection rates of Down's syndromeall ages
The overall antenatal detection rate was 171/323 (53%, 48% to 58%). Table 2 shows the proportion detected antenatally in each district and the screening methods that prompted an offer of an invasive procedure. There was no significant advantage to any screening policy, and the addition of more screening tests did not produce an additive effect.

In 1993, when serum screening was introduced to districts A and B, the uptake was about 85%, but by 1999 this had dropped to 55% in district A and 65% in district B. As a result, only 24% of affected fetuses were detected after serum screen results that indicated a high risk (table 2). The remainder were detected as a result of other indications for invasive testing. Some women aged over 35 years opted directly for amniocentesis, and in others an abnormal scan result led to the diagnosis of Down's syndrome. In these districts, among women who accepted serum screening its sensitivity was 43% in women aged under 35 years and 80% in women over 35 years.

Detection rates in women aged under 35 years
One of the arguments advanced for methods such as nuchal translucency and serum screening is that they would increase the detection rate of Down's syndrome in young women. The overall detection rate in women aged under 35 was 47/137 (34%, 26% to 42%). There were differences between the districts but these were not significant (see the full version of this paper on bmj.com).

There were 90 cases that were not diagnosed before 24 weeks' gestation in women under 35 years. In 57 there was no indication for invasive testing under the local policy, though nine fetuses had unrecognised heart abnormalities that were detected postnatally. In the 33 other cases, seven women had refused serum screening, 15 women had false negative results on serum screening and six had false negative results on nuchal scanning, three women declined invasive testing after nuchal or serum screens that indicated high risk, and two cases were in twin pregnancies with one affected fetus.

Detection rates in women aged 35 years and over
Districts that screened by maternal age might be expected to detect all cases in women aged 35 years, whereas other screening methods would miss some of these cases. Across all districts, 186 cases of Down's syndrome occurred in older women and 124 (67%, 60% to 74%) were detected antenatally. Of the 62 missed antenatally, in 10 (16%) test results were falsely negative (three serum screen, seven nuchal scans), in 43 (69%) women declined antenatal diagnosis, and in five (8%) invasive testing was declined as the pregnancy was twin. Finally, four (7%) in women aged 35 were undetected in district H, where the policy was to offer invasive testing only to women over 37 years. Among all women 35 years and over who had affected fetuses, 23% (43/186) refused a diagnostic test.

Invasive procedure rates
The rates of amniocentesis and chorionic villus sampling varied from 2.8% in the district with the youngest maternal population (D) and 4.2% in district H, where amniocentesis is offered to women aged 37 years and over, to 7.7% in district E, which had the oldest maternal population. District F is a referral centre and its rate is raised by having cases referred from other districts. The invasive procedure rate for local women in district F averaged 5.4%. Thus about 1.4% of invasive procedures were performed on women referred from elsewhere.

	Discussion

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We found no evidence that serum and nuchal translucency screening improves antenatal detection rates or reduces rates of invasive procedure. Our findings suggest that the recently announced government initiative to introduce universal serum screening from 2004 will not achieve its stated objectives. The current maternal age distribution observed in our study is different to that used in the demonstration projects; 15% of women who were pregnant during the study period were aged 35 years and over. As a consequence 58% of babies with Down's syndrome were born to women in this age group. This shows a return to levels seen in the 1950s and 1960s, when over 13% of childbearing women were aged 35 and over and more than half of the children with Down's syndrome were born to women in this age group.² In districts with a higher proportion of older women the use of maternal age detects a high proportion of affected fetuses. The addition of routine anomaly scans, which are already offered in most UK health districts, also allows a large proportion of affected fetuses to be detected in younger women.

To avoid continuing the confusion that Down's screening currently causes in pregnant women, we believe that new screening methods should be offered only as part of a controlled study until their benefit is proved.

	Acknowledgments

   Contributors: See bmj.com

	Footnotes

Funding: None.

Competing interests: None declared.

The full version of this article appears on bmj.com

	References

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1.	Mutton DE, Alberman E, Ide R, Bobrow M. Results of first year (1989) of a national register of Down's syndrome in England and Wales. BMJ 1991; 303: 1295-1297.
2.	Adams MM, Erickson JD, Layde PM, Oakley GP. Down's syndrome. Recent trends in the United States. JAMA 1981; 246: 758-760[Abstract/Free Full Text].

(Accepted 3 December 2001)