Monochorionic, Dizygotic, Sex Discordant Twins With Twin Anemia Polycythemia Sequence

Monochorionic twins are typically monozygotic with identical fetal sex. We present a monochorionic, diamniotic twin pair of a triplet gestation with discordant fetal sex also affected with twin anemia polycythemia sequence (TAPS). The pregnancy was conceived with ovulation induction, and the initial ultrasound at 8 weeks revealed a dichorionic triamniotic triplet gestation. Discordant sex between the monochorionic pair was noted on ultrasound at 20 weeks, with Fetus 1 appearing phenotypically female and Fetus 2 appearing phenotypically male. At 26 weeks, the middle cerebral artery peak systolic velocity discordance between the monochorionic pair met criteria for stage II TAPS, a condition produced by unequal sharing of red blood cells across small diameter vascular anastomoses on the monochorionic placenta. Subsequent fetal blood sampling demonstrated hemoglobin of 8.2 g/dL of the donor twin, Fetus 2, and intrauterine transfusion was performed. Rapid progression to stage III TAPS was noted at 29 weeks, and delivery was recommended. The triplets were delivered via uncomplicated primary cesarean delivery. Triplet 1 had female genitalia at birth and Triplet 2 had male genitalia. Placental histology demonstrated a dichorionic, triamniotic placenta confirming monochorionicity for Fetus 1 and 2. Fluorescence in situ hybridization of the nucleated peripheral blood cells for Triplet 1 demonstrated 61% XX and 39% XY. In Triplet 2, FISH demonstrated 56% XX and 44% XY. Chromosomal microarray analysis conducted from buccal swabs collected 6 days after birth revealed that the sex chromosome complement for triplet 1 matched the female appearing genitalia {XX [arr(X,1-22)x2]} and for triplet 2 matched the male genitalia {XY [arr(X,Y)x1, (1-22)x2]}, with no mosaicism. Results confirm a trizygotic triplet pregnancy with a functionally monochorionic placenta for the twin pair presenting with TAPS, with direct evidence of shared blood across in utero anastomosis on the placenta. This rarely reported phenomenon may be secondary to close implantation or fusion of distinct embryos with the development of vascular anastomoses.