NEW ESTIMATES OF DOWN-SYNDROME RISKS AT CHORIONIC VILLUS SAMPLING, AMNIOCENTESIS, AND LIVEBIRTH IN WOMEN OF ADVANCED MATERNAL AGE FROM A UNIQUELY DEFINED POPULATION

Current measures of livebirth prevalence of Down syndrome are derived from data obtained up to 20 years ago, before the introduction of the prenatal diagnostic tests amniocentesis and chorionic villus sampling (CVS). For women aged 36-52 years, but who were not tested prenatally, we proposed to make a direct estimate of current livebirth prevalence of Down syndrome. We could also determine prevalence at the time of C VS and amniocentesis in women of the same age undergoing prenatal test ing. Differences in these prevalences allow an estimation of the relat ive loss of Down syndrome during pregnancy. In Victoria, Australia, we identified 3041 women having CVS, 7504 having amniocentesis, and 13 1 39 having no test. Smoothed regression estimates of age-specific liveb irth prevalence were found to be higher than in the early studies. The estimate of spontaneous loss was 17 per cent between the time of CVS and amniocentesis, and is per cent after the time of amniocentesis. Th e latter figure is lower than previous estimates and may be explained by a greater likelihood of a Down syndrome fetus surviving to be liveb orn, given the modern approach to early obstetric intervention. These current risk estimates of livebirth may be useful updates for genetic counselling, but perhaps more importantly, may be used as precise mate rnal age-related risk figures, necessary in the design and implementat ion of prenatal screening programmes for Down syndrome.