Trisomy recurrence: a reconsideration based on North American data.

Few reliable data exist concerning the recurrence risk for individual trisomies or the risk for recurrence of trisomy for a different chromosome. We collected records from two sources: (1) prenatal diagnoses performed at the Hopital Sainte-Justine in Montreal and (2) karyotype analyses performed at Genzyme. Using the standardized morbidity ratio (SMR), we compared the observed number of trisomies at prenatal diagnosis with the expected numbers, given maternal age-specific rates (by single year). SMRs were calculated both for recurrence of the same trisomy (homotrisomy) and of a different trisomy (heterotrisomy). After all cases with an index trisomy 21 were combined, the SMR for homotrisomy was 2.4 (90% CI 1.6-3.4; P=.0005). For women with both the index trisomy and subsequent prenatal diagnosis at age <30 years, the SMR was 8.0; it was 2.1 for women with both pregnancies at age >/=30 years. For the other index viable trisomies (13, 18, XXX, and XXY) combined, the SMR for homotrisomy was 2.5 (90% CI 0.7-8.0). For heterotrisomy, the SMR after an index trisomy 21 was 2.3 (90% CI 1.5-3.8, P=.0007); the SMR did not vary with maternal age at the first trisomy. When all cases with index viable trisomies were combined, the SMR for heterotrisomy was 1.6 (90% CI 1.1-2.4; P=.04). For prenatal diagnoses following a nonviable trisomy diagnosed in a spontaneous abortion (from Genzyme data only), the SMR for a viable trisomy was 1.8 (90% CI 1.1-3.0; P=.04). The significantly increased risk for heterotrisomy supports the hypothesis that some women have a risk for nondisjunction higher than do others of the same age.