Objectives: To assess the accuracy of non-invasive prenatal testing (NIPT) for sex chromosome aneuploidy (SCA) in routine clinical practice and to review counselling and sonographic issues arising in SCA cases. Methods: Three specialist Australian obstetric ultrasound and prenatal diagnosis practices offering NIPT after 10 weeks' gestation participated in this study. NIPT was reported for chromosomes 21, 18, 13, X, and Y. Results: NIPT screening was performed in 5,267 singleton pregnancies. The odds of being affected given a positive screening result (OAPR) was lowest for SCAs, most notably for monosomy X (20%). Fewer women underwent invasive prenatal testing when counselled regarding a high risk for SCA (65.5%) compared with those who had a high risk for another aneuploidy (85%). The positive screening rate of NIPT including SCA was 2.3%, but 1.2% if only the autosomal trisomies were included in the panel. Conclusion: The addition of SCA testing to NIPT doubles the positive screening rate. The OAPR for SCAs (most notably for monosomy X) is reduced compared with the autosomal trisomies. Clinicians need a more extensive discussion with women prior to the inclusion of the X and Y chromosomes in the NIPT panel, given the complexity in counselling regarding further management and the additional anxiety that these abnormal results may cause. A benefit of sex chromosome analysis is an improvement in antenatal diagnosis of some disorders of sexual development.

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