Objectives-To evaluate the effectiveness of the first 7 years of a programm
e for second trimester antenatal screening for Down's syndrome, using alpha
-fetoprotein (alpha FP) and total human chorionic gonadotropin (hCG) as ma
ternal serum markers.
Methods-A clinical biochemistry laboratory providing a screening service fo
r four obstetric units. Women attending for antenatal care who accepted an
offer of serum screening for Down's syndrome were tested between 15 and 20
weeks gestation. Down's risk estimates were calculated using maternal serum
alpha FP and total hCG results as modifiers of the maternal age related ri
sk. Outcome was determined in collaboration with the regional cytogenetics
unit.
Results-In 7 years 66 631 women were screened, in whom 108 Down's syndrome
pregnancies were identified. Risks for Down's syndrome were reported withou
t a specified cut off recommendation; however, at a cut off of 1 in 250, 72
(66.7%) of the affected pregnancies were screen positive, the false positi
ve rate was 5.8%, and the uptake of amniocentesis 71.2%. The detection rate
was higher in women screened before 17 weeks (70.5%) than in those screene
d at 17 weeks or later (56.7% overall and 20.0% in those under 30 years). T
he uptake of screening declined gradually from 84% in 1992 to 59.8% in 1998
.
Conclusions-Two marker screening using alpha FP and total hCG is an effecti
ve way of screening for Down's syndrome and is widely accepted in the local
community. Detection rates were comparable with other second trimester stu
dies using two markers including both total and free beta hCG.