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(Abstracted from Ultrasound Obstet Gynecol 2020;55:502–509) Structural anomalies are frequently detected by prenatal ultrasound, and understanding the cause of such anomalies is important for genetic counseling and prenatal management. Chromosomal microarray analysis (CMA) has been shown to increase the rate of genetic diagnosis compared with karyotyping.

Objectives To evaluate the incidence and types of chromosomal abnormalities detected in twins with structural anomalies and compare their distribution according to chorionicity and amnionicity and by structural‐anomaly type. The added value of chromosomal microarray analysis (CMA) over conventional karyotyping in twins was also estimated. Methods This was a single‐center, retrospective analysis of 534 twin pregnancies seen over an 11‐year period, in which one or both fetuses were diagnosed with congenital structural anomalies on ultrasound. The ultrasound findings and invasive prenatal diagnostic results were reviewed. Twin pregnancies were categorized as monochorionic monoamniotic (MCMA), monochorionic diamniotic (MCDA) or dichorionic diamniotic (DCDA). Chromosomal abnormalities detected by G‐banding karyotyping and/or CMA were analyzed by chorionicity and amnionicity and by structural‐anomaly type. Results The 534 twin pairs analyzed comprised 25 pairs of MCMA, 112 pairs of MCDA and 397 pairs of DCDA twins. Of the 549 fetuses affected by structural anomalies, 432 (78.7%) underwent invasive prenatal testing and cytogenetic results were obtained. The incidence of overall chromosomal abnormalities in the DCDA fetuses (25.4%) was higher than that in the MCMA (3.7%) and MCDA (15.3%) fetuses. The incidence of aneuploidy was significantly higher in the DCDA group (22.8%) than in the MCMA (0.0%) and MCDA (12.4%) groups. The incidence of chromosomal abnormalities detected in fetuses, with anomalies of the cardiovascular, faciocervical, musculoskeletal, genitourinary and gastrointestinal systems, was higher in the DCDA group than in the MCDA group. In both the DCDA and MCDA groups, hydrops fetalis was associated with the highest incidence of chromosomal abnormality; of these fetuses, 67.6% had Turner syndrome (45,X). Pathogenic copy‐number variations (CNVs) undetectable by karyotyping were identified by CMA in five (2.0%; 95% CI, 0.3–3.7%) DCDA fetuses. No pathogenic CNVs were found in MCMA and MCDA twins. Conclusions Dichorionic twins with structural anomalies have a higher risk of chromosomal abnormalities, especially aneuploidies, than do monochorionic twins. The incremental diagnostic yield of CMA over karyotyping seems to be lower (2.0%) in twins than that reported in singleton pregnancy. Copyright © 2019 ISUOG. Published by John Wiley & Sons Ltd.

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