Fetal karyotyping by chorionic villus sampling after the first trimester

Objective To evaluate chorionic villus sampling (CVS) as a technique for ka ryotyping after the first trimester by examining the incidence of result fa ilure, confined placental mosaicism, and false positive or negative results at different gestational ages. Methods During a nine year period between 1989 and 1997, all results of CVS between 8 and 37 weeks of gestation provided by the Regional Cytogenetics Centre were analysed retrospectively by examining indications for CVS, weig hts of tissue received, gestational age at sampling and karyotype results. Results There were 2424 chorionic villus samples analysed by the direct met hod and/or cell culture. In 1548 cases CVS was performed before 14 weeks (G roup 1), in 685 between 15 and 20 weeks (Group 2), in 160 between 21 and 28 weeks (Group 3) and in 31 cases after 29 weeks (Group 4). Although there w as a trend for an increasing rate of failed direct preparation results from Groups 1 to 4 which were 3.8%, 4.7%, 5.6% and 6.6%, respectively; these re sults were not significantly different. There were 19 cases of confined pla cental mosaicism and the incidence was significantly greater in Group 3 com pared with Group 1 (P < 0.05), and in Groups 3 and 4 combined compared with Group 1. There were six false positive and one false negative result follo wing direct analysis with no significant differences between gestational ag es. Conclusions CVS is a useful test after the first trimester, especially when a fast result is clinically required. However, after 20 weeks, when cordoc entesis is available, the higher rate of cytogenetic discordancy between th e placenta and the fetus means that cordocentesis may be preferable.