INTRAUTERINE LETHALITY OF TRISOMY-21 FETUSES WITH INCREASED NUCHAL TRANSLUCENCY THICKNESS

This study investigates whether first-trimester screening for trisomy 21 by fetal nuchal translucency thickness preferentially identifies th ose fetuses destined to die in utero and examines the potential impact of such a method of screening on the live birth incidence of trisomy 21. In 70 pregnancies, trisomy 21 was diagnosed at 12 (range 11-14) we eks of gestation and the parents opted for elective termination which was carried out at 14 (12-20) weeks. In all cases, viability was estab lished by ultrasound scan at the rime of chorion villus sampling (CVS) and just before termination of pregnancy. Eight (11.4%) fetuses died in the interval between CVS and termination of pregnancy and this rate of lethality was higher than the 6.9% estimated rate for an unselecte d population of trisomy 21 fetuses. This 4.5% increase may, in part, b e attributed to the effects of CVS and may also be due to patient sele ction on the basis of increased nuchal translucency. The rate of letha lity increased with translucency thickness from 5.3% for those with tr anslucency of 1-3 mm to 23.5% for translucency of > 7 mm. In trisomy 2 1, the rate of intrauterine lethality is associated with nuchal transl ucency thickness. Nevertheless, a policy of screening by maternal age and fetal nuchal translucency followed by selective termination of aff ected fetuses would still result in a move than 70% reduction in the L ive birth incidence of trisomy 21.