Background Trisomy 18 (T18) and trisomy 13 (T13) are lethal autosomal trisomies characterised by major structural anomalies. New NSC standards1 include a requirement to audit detection rates.
Methods Cases of cytogenetically confirmed T18 and T13 were selected using a regional, population-based, multiple source congenital anomaly register covering a birth cohort of 995 003, and were analysed in three sequential 5-year cohorts.
Results The cohort included 542 cases of T18 and 244 of T13, giving a total prevalence (per 10 000 births) of 5.46 for T18 and 2.45 for T13. The prevalence of T18 increased from 3.98 in 1995–1999 to 6.89 in 2004–2009 (p<0.001), in line with the increasing age of the maternal population. Live birth prevalences were 0.73 and 0.46/10 000 live births for T18 and T13 respectively and did not vary over time.
Prenatal detection rates increased but only reached the 95% standard in 2005–2009 (T18: 95.0%, T13: 96.3%). Uptake of TOP following a prenatal karyotype was 84.8% and did not change. The majority (58.3%) of affected pregnancies in 2005–2009 were karyotyped or had ended before 18 weeks.
Conclusions Although the prevalence of T18 is increasing, the combined impact of better detection and high TOP rates means that the live birth prevalences for both trisomies have not changed. However as a result, the workload for fetal medicine services has increased substantially.
The observed trend toward earlier diagnosis suggests that the current screening programme's focus on the fetal anomaly scan for detection of T18 and T13 is out of step with practice.
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