Medical Genetics D, Sex Approximately 80% of cases occur in females. The preponderance of females with trisomy 18 in liveborn infants(sex ratio 0.63) compared to fetuses with prenatal diagnoses(sex ratio 0.90) indicates a prenatal selection against males with trisomy 18 after the time of amniocentesisMedical Genetics D. Sex Approximately 80% of cases occur in females. The preponderance of females with trisomy 18 in liveborn infants (sex ratio 0.63) compared to fetuses with prenatal diagnoses (sex ratio 0.90) indicates a prenatal selection against males with trisomy 18 after the time of amniocentesis