ANALYSIS OF MATERNAL SERUM ALPHA-FETOPROTEIN AND FREE BETA-HUMAN CHORIONIC-GONADOTROPIN IN THE FIRST TRIMESTER - IMPLICATIONS FOR DOWNS-SYNDROME SCREENING

The aim of this study was to determine the maternal population, pregna ncy, serum alpha-fetoprotein (AFP) and free beta subunit of human chor ionic gonadotrophin (F beta hCG) parameters in a large series of women attending prenatal clinics before 15 weeks' gestation and to assess t he practical problems of population screening for Down's syndrome in t he first trimester using these markers. Serum samples were collected f rom 8600 women attending prenatal clinic booking visits. Maternal seru m AFP and F beta hCG medians were calculated for each day of gestation (49-104 days), using both dates and ultrasound estimates of gestation . The effects of maternal weight, twin pregnancies, and threatened abo rtion on AFP and F beta hCG levels were analysed. The median age of th e population was 27.1 years and the median weight 62.1 kg. Twenty-six per cent of samples were collected before 70 days and 50 per cent befo re 78 days' gestation. Eighty-nine per cent of all samples had gestati onal estimates by dates, 60 per cent by ultrasound and 52 per cent by both dates and ultrasound. The AFP median was 5 kU/l at 49 days, 5.9 k U/l at 70 days, and 17.9 kU/l at 100 days. The peak median F beta hCG level was 66.4 ng/ml at 64 days, falling to 20.6 ng/ml at 100 days' ge station. Both AFP and F beta hCG levels showed log Gaussian distributi ons but the standard deviation for AFP was 20 per cent greater than th at found in the second trimester. AFP and F beta hCG levels showed an inverse relationship with maternal weight and were increased in twin p regnancies (1.68 and 1.97 multiples of the median, respectively). AFP and F beta hCG can be readily measured in a large screening population in the first trimester. Down's syndrome screening protocols based on these markers could be refined by the use of gestations in individual days but AFP is likely to be a less effective marker and detection rat es are likely to be lower than in the second trimester. To realize the potential of first-trimester screening, more women should be encourag ed to attend the prenatal clinic in early pregnancy and ultrasound dat ing should be carried out for all pregnancies at this stage.