Screening for Down's syndrome: the first two years experience in Bristol.

OBJECTIVES

To evaluate the effectiveness of a programme for antenatal screening for Down's syndrome using alpha fetoprotein and total human chorionic gonadotrophin as maternal serum markers.

SETTING

A district general hospital providing a screening service to a local purchasing authority and (under contract) to another purchasing authority in the same region.

METHODS

Patients were counselled and screened between 15 and 20 weeks gestation and Down's risk estimates calculated using the maternal serum marker results as modifiers of the age related risk. Outcome was determined in collaboration with the Regional Cytogenetics Unit.

OUTCOME MEASURES

Detection rate for Down's syndrome, false positive rate, uptake of screening, and uptake of amniocentesis.

RESULTS

In two years 22816 women were screened (approximately 84% of population); 32 Down's pregnancies were identified, 19 (59.4%) had a reported risk of > or = 1:250 and 20 (62.5%) a reported risk of > or = 1:300. Of those screened before 17 weeks, 16/20 (80%) had a reported risk of > or = 1: 300 compared with 4/12 (33%) of those screened later (P = 0.008); 4.64% of patients screened had reported risks > or = 1: 250 and 5.87% reported risks of > or = 1:300. Amniocentesis uptake was 70% in patients with reported risks of > or = 1:300.

CONCLUSIONS

Overall the screening programme was effective but screening before 17 weeks was very much more effective than screening later.