Objectives: To compare the maternal age, sex ratio, and associated major anomalies among fetal trisomy 18 cases with different cell division of error. Methods: Thirty-one consecutive cases of fetal trisomy 18 detected perinatally during a period of 6 years were studied. Among these, 18 were 47,XY,+18, and 13 were 47,XX,+18. The average gestational age at diagnosis was 19.7 ± 4.6 weeks, and the maternal age at diagnosis was 34.5 ± 5.8 years. DNA polymorphism analysis was applied to determine the parental origin, stage of non-disjunctional error and recombination. Results: Twenty-eight cases were of maternal origin. Among these, 20 had major anomalies, 17 had meiosis II (MII) errors, 10 had meiosis I (MI) errors, and one had a postzygotic mitotic (PZM) or non-crossover MII error. Three cases were of paternal origin. Among these, two had major anomalies, two had MI errors, and one had a PZM or non-crossover MII error. For the 17 cases with maternal MII errors, the average maternal age was 34.5 ± 6.6 years. Of these cases, 12 had major anomalies, 13 were male, and 4 were female, giving a male:female sex ratio of 3.25:1. For the 10 cases with maternal MI errors, the average maternal age was 34.8 ± 5.7 years. Of these cases, seven had major anomalies, three were male, and seven were female, giving a male:female sex ratio of 0.429:1. Conclusion: In trisomy 18, there is a male preponderance in the fetuses caused by maternal MII errors and a female preponderance in the fetuses caused by maternal MI errors. No significant difference was noted in maternal age or in associated major anomalies between the two groups of maternal MII errors and maternal MI errors. No significant difference was noted in associated major anomalies between the maternal and paternal cases.
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