A 37 year old Asian woman G2P0010 with a known twin gestation at 30 weeks 2 days by in vitro fertilization dating, immigrated to the United States from China to continue her prenatal care. She had three embryos conceived by intracytoplasmic sperm injection (ICSI) that were transferred on day 3. First-trimester ultrasound reports from China revealed monochorionic diamniotic twin pregnancy. On presentation, to our office, an ultrasound showed Twin A with male genitalia and Twin B with female genitalia and a thin dividing membrane which was consistent with monochorionicity. Placental pathology also confirmed monochorionicity. This case explores a potential mechanism for differing fetal sexes in a case of monochorionic-diamniotic twins due to fusion chimera formation. Chimera formation has been thought to occur via disruption of the zona pellucida during the ICSI process and transfer of cells between different embryos. As assisted reproductive technologies become more prevalent, exploring the developmental pathogenesis and genetics of twinning is warranted to better understand discordant findings from prenatal ultrasound and genetics. The interactions of the transferred embryos could lead to sustained stem cell chimerism that is present after birth.
Keywords: Amnion, Chimera, Chorionicity, Embryos, Fetal sex, Intracytoplasmic Sperm Injection, Mosaicism