• Zhiyong Zou, Linhuan Huang, Shaobin Lin, Zhiming He, and Yanmin Luo.
• Department of Obstetrics & Gynecology, The First Affiliated Hospital of Sun Yat-Sen University, Guangzhou, China.
• Prenat. Diagn. 2018 May 1; 38 (6): 428-434.

ObjectiveTo evaluate the incidence and characteristics of unusual twinning by using single nucleotide polymorphism (SNP) array to identify twin zygosity.MethodsThis study reviewed 386 twin pairs who were seen for prenatal or postnatal diagnosis and underwent SNP array to detect zygosity.ResultsThe incidence of monozygotic (MZ) twins was 11.36% (25/220) in the assisted reproductive technology (ART)-conceived group. Monochorionic dizygotic twins represented 3 of 24 monochorionic ART-conceived twin pairs (3/24, 12.50%) but none in the spontaneous twin pairs. Among 4 single-embryo transfer twin pairs, 3 represented unusual twinning, including 2 MZ twin pairs with discordant karyotypes and 1 dizygotic twin pair of the same gender. Of the pregnancies with 2 or more embryos transferred, 7.77% (15/193) were MZ. Additionally, there was a dichorionic monozygotic twin pair with placental vascular anastomoses from a day-5 blastocyst transfer.ConclusionSingle nucleotide polymorphism array can provide zygosity diagnosis in addition to chromosomal copy number variation and uniparental disomy detection. ART twin pregnancies have a risk of unusual twinning, such as monochorionic dizygotic, single-embryo transfer twin pairs with discordant karyotypes or dizygotic, and dichorionic monozygotic with vascular anastomoses from day-5 transfer.© 2018 John Wiley & Sons, Ltd.

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