A Case of Sex Discordant Dichorionic Diamniotic Twins after Single Embryo Transfer and the Importance of Zygosity Testing

Single embryo transfer (SET) is a technique used in assisted reproductive treatment (ART) that is used to promote singleton pregnancies. To date, there are five reported cases of dizygotic twin pregnancies with mothers who underwent SET. Here, we present a sixth case of a dichorionic, diamniotic twin pregnancy with sex discordance. The patient is a 34-year-old woman with unexplained secondary infertility who underwent in vitro fertilization (IVF) and frozen-thawed embryo transfer from a SET. The ultrasonographic images from the first and second trimester scans identified dichorionic, diamniotic twin gestations. The delivery was full term and postnatal genetic testing confirmed 46, XX, and 46, XY offspring. Pathology reports of the placental and membrane findings reported diamniotic, dichorionic twins. There was no zygosity testing conducted, thus it is unknown if the twins are monozygotic or dizygotic. Two possible etiologies for sex-discordant twins, in this case, are concurrent natural conception via breakthrough ovulation at the time of SET, or discordant postzygotic nondisjunction of a single embryo. Multiple gestations may still occur in the setting of SET and zygosity testing in these instances would better elucidate our understanding of this occurrence. Moreover, improved data on the zygosity of multiple gestations following SET may enhance patient counseling.