Recurrence risks for trisomies 13, 18, and 21.

The objective was to establish whether the risk of trisomies 13, 18, and 21 (Patau, Edwards, and Down syndrome, respectively) in a subsequent pregnancy is raised for women who have had a previous pregnancy with trisomy 13, 18, or 21. Birth defect register data were used to investigate this issue. Pregnancy data from three Australian population-based birth defect registers contained 5,906 women with a previous trisomy 13, 18, or 21 pregnancy in whom there were 3,713 subsequent pregnancies, 75 of which were trisomic. Relative risk of subsequent trisomy at 15 weeks gestation was estimated by comparing the observed number of subsequent trisomies with the expected number of subsequent trisomies based on maternal age-related risk. There was evidence of increased risk of the same trisomy subsequent to a previous pregnancy with trisomy 13 or 18 (RR = 3.8 (1.5, 7.9)), the increase in risk being greater for women aged under 35 at the previous trisomic pregnancy (RR = 7.8 (2.1, 20.2)). There was also evidence of increased risk of trisomy 21 subsequent to previous trisomy 21 (RR = 2.2 (1.6, 2.9)), again higher in women under 35 at previous affected pregnancy (RR = 3.5 (2.1, 5.5)). There was a suggestion that the risk of a different trisomy subsequent to trisomy 21 may also be increased (RR = 1.4 (0.7, 2.5)). In conclusion, women who have had a previous trisomic pregnancy, particularly those under 35 years of age at the time, appear to be at an increased risk of future pregnancies being trisomic.