Abstract
Two equations are given to estimate the risk of a woman having a Down's syndrome pregnancy according to maternal age: one for use in estimating antenatal screening performance and the other for use in estimating an individual woman's risk of having an affected pregnancy. Because Down's syndrome pregnancies have an increased tendency to result in a spontaneous fetal loss, the estimation of a woman's risk of having an affected pregnancy will be dependent on gestational age. The best estimates of the prevalence of Down's syndrome are obtained from information relating to births and can reliably be adjusted to prevalence in early second trimester. The most reliable estimates of risk of a Down's syndrome pregnancy using all the currently used markers apply to early in the second trimester. These risks cannot accurately be adjusted to apply to term, because the first trimester markers have not in general been studied in pregnancies that continue to term. Therefore the early second trimester of pregnancy is the gestational age at which all screening performances should be given for the different antenatal screening programmes for Down's syndrome.