First trimester prenatal diagnosis of trisomy 21 in discordant twins using fetal nuchal translucency thickness and maternal serum free beta-hCG and PAPP-A.

Screening for trisomy 21 in twin pregnancies poses a serious clinical, ethical and moral dilemma (Reynolds, 1995), leading some authors to advocate that such screening should be discouraged (Wald et al., 1997). Nevertheless, algorithms have been developed (Wald et al., 1991; Spencer et al., 1994) for biochemical screening in twins during the second trimester which will allow detection of approximately 50% of cases when one twin is unaffected and the other affected (discordant twins). This theoretical model has been shown to work in routine screening practice (Verdin et al., 1997). Signi®cant interest is now focussed on moving screening for trisomy 21 into the ®rst trimester, when a combination of fetal nuchal translucency, maternal serum free b-hCG and PAPP-A have been shown to identify 90% of cases of trisomy 21 (Spencer et al., 1999) and 90% of other chromosomal anomalies (Tul et al., 1999; Spencer et al., 2000a, b, c). Sebire et al. (1996a) showed that fetal nuchal translucency could be used to screen both concordant and discordant twins for trisomy 21; and, more recently, data on the distribution of maternal serum biochemical markers has led to the development of a combined algorithm for screening twins in the ®rst trimester (Spencer, 2000). Here we present details of our ®rst case of twins in which trisomy 21 was prospectively detected and diagnosed.