FIRST TRIMESTER BIOCHEMICAL SCREENING FOR DOWNS-SYNDROME

A number of biochemical markers in maternal serum have been proposed for first trimester screening for Down's syndrome. The most promising four are pregnancy associated plasma protein A (PAPP-A), the free beta sub-unit of human chorionic gonadotrophin (hCG) (free beta glycoprotein sub-unit), unconjugated oestriol (uE(3)) and alpha-fetoprotein (AFP). An analysis of the published literature suggests that 70% of affected pregnancies could be detected for a 5% false-positive rate if the four markers are used in combination with maternal age and assumed to be independent measures of risk. This is a level of performance that is similar to second trimester screening. It is, however, a tentative estimate because of the assumption of independence and the possibility that the effect may be exaggerated by publication bias. Further research is needed before such screening is introduced. Other first trimester markers which have been studied include total hCG, free alpha-hCG, CA125, PLAP and SP1 but they either look unpromising or there are too few data available to determine their value. The timing of antenatal diagnosis by means of chorion villus sampling should be delayed until after 10 weeks of pregnancy because of the risk of causing limb defects. Screening need not, therefore, be performed before about 9 or 10 weeks of pregnancy.