Fetal karyotyping by chorionic villus sampling after the first trimester.

OBJECTIVE

To evaluate chorionic villus sampling (CVS) as a technique for karyotyping after the first trimester by examining the incidence of result failure, 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, weights of tissue received, gestational age at sampling and karyotype results.

RESULTS

There were 2424 chorionic villus samples analysed by the direct method and/or cell culture. In 1548 cases CVS was performed before 14 weeks (Group 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 was 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 results were not significantly different. There were 19 cases of confined placental mosaicism and the incidence was significantly greater in Group 3 compared 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 following direct analysis with no significant differences between gestationar ages.

CONCLUSIONS

CVS is a useful test after the first trimester, especially when a fast result is clinically required. However, after 20 weeks, when cordocentesis is available, the higher rate of cytogenetic discordancy between the placenta and the fetus means that cordocentesis may be preferable.