Trisomy 13 and trisomy 18 in a defined population: epidemiological, genetic and prenatal observations.

OBJECTIVES: To establish precise incidence figures for trisomy 13 and trisomy 18 in the former Trent region, to identify current prenatal diagnostic practice, and to assess the potential impact of the introduction of recently devised prenatal diagnostic practices. METHODS: An audit of all cases of trisomy 13 and trisomy 18 ascertained through the records of the Trent Congenital Anomalies Register and the Trent Regional Cytogenetic Laboratories. RESULTS: Forty-four cases of trisomy 13 and 88 cases of trisomy 18 were ascertained. Advanced maternal age effects were observed. Of all cases, 64% were first detected through chromosomal analysis initiated because of abnormalities noted on fetal anomaly scanning in the second trimester, whereas only 3% of cases were detected through the serum-screening programme currently offered for Down syndrome. In 11% of cases, the diagnosis was first suspected after birth. Twelve percent of couples chose to continue pregnancy following chromosomal confirmation of a suspected diagnosis. CONCLUSION: The introduction of a highly sensitive prenatal diagnostic screening programme would have a major impact on the timing and proportions of all trisomy 13 and 18 cases diagnosed in pregnancy as gauged by current practice. It is important that health professionals involved in prenatal counselling be aware that, as with Down syndrome and anencephaly, around 12% of prospective parents of a child with trisomy 13 or 18 choose to continue rather than terminate the pregnancy.