KARYOTYPE ABNORMALITIES IN FETUSES DIAGNOSED AS ABNORMAL ON ULTRASOUND BEFORE 20 WEEKS GESTATIONAL-AGE

This study examined rates of karyotype abnormalities in fetuses diagno sed by ultrasound as abnormal before 20 weeks' gestational age and whi ch prompted a follow-up amniocentesis or chorionic villus sampling. Th ose diagnosed before 20 weeks were compared with those diagnosed at or after 20 weeks. A retrospective study identified ultrasonographically abnormal fetuses in whom karyotyping had been undertaken, 306 fetuses before 20 weeks' gestational age and 241 after. Isolated malformation s before 20 weeks had, on average, an 18 per cent risk of karyotype ab normality, compared with 20 per cent later. Specific rates were calcul ated; for example, heart abnormality was associated with karyotype abn ormality in 7 per cent of cases before 20 weeks and in 14 per cent lat er. Multiple malformations and karyotype abnormalities were found toge ther in 28 per cent of fetuses prior to 20 weeks and in 33 per cent of the older fetuses. Specific associations included nuchal oedema and t risomy 21 in 21 per cent of fetuses before 20 weeks. No karyotype abno rmalities were found in fetuses diagnosed with choroid plexus cysts. A n overview of trisomies in Victoria, in 1991, showed that 50 per cent of trisomy 18, 42 per cent of trisomy 13, and 9.5 per cent of trisomy 21 cases were identified by ultrasound in women less than 37 years of age. Another 2.86 per cent of trisomy 21 fetuses were detected in wome n of advanced maternal age who underwent amniocentesis or chorionic vi llus sampling, making a total of 3.81 per cent of trisomy 21 that were detected prenatally. The importance of early karyotyping specifically relates to the ongoing management of the pregnancy if the chromosomes are normal, and facilitates decision-making regarding termination of pregnancy if the chromosomes are abnormal.