Objective: To study prevalence of Turner's syndrome in Denmark and to assess validity of prenatal diagnosis.
Design: Study of data on prenatal and postnatal Turner's syndrome in Danish Cytogenetic Central Register.
Subjects: All registered Turner's syndrome karyotypes (100 prenatal cases and 215 postnatal cases) during 1970-93.
Main outcome measures: Prevalence of Turner's syndrome karyotypes among prenatally tested fetuses and Turner's syndrome among liveborn infants.
Results: Among infant girls, prevalence of Turner's syndrome was 32/100,000. Among female fetuses tested by amniocentesis, prevalence of Turner's syndrome karyotypes was 176/100,000 (relative risk of syndrome, 6.74 compared with prevalence among untested pregnancies). Among female fetuses tested by chorion villus sampling, prevalence of syndrome karyotypes was 392/100,000 (relative risk, 16.8). We excluded prenatal tests referred because of results of ultrasound scanning: among fetuses tested by amniocentesis revised relative risk was 5.68, while revised relative risk among fetuses tested by chorion villus sampling was 13.3. For 29 fetuses with prenatal diagnosis of possible Turner's syndrome, pregnancy was allowed to continue and 24 children were live born. Thirteen of these children were karyotyped postnatally, and diagnosis of Turner's syndrome had to be revised for eight, seven being normal girls and one boy. This gives tentative predictive value of amniocentesis in diagnosing Turner's syndrome of between 21% and 67%. There was no significant relation between mother's age and risk of Turner's syndrome.
Conclusions: Discrepancy between prenatal and postnatal prevalence of Turner's syndrome challenges specificity of prenatal examination in diagnosing Turner's syndrome.