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 approx...
