Trizygotic dichorionic triplets with 46,XX/46,XY chimerism in both fetuses of the monochorionic pair

Monochorionic fetuses are traditionally thought of as monozygotic (MZ), arising from a single embryo that splits during early embryogenesis. With rare exception, they typically exhibit the same genotype and the same gender. However, several recent reports have documented that monochorionicity does not always indicate monozygosity (Souter et al., 2003). At least 14 such cases have been reported (Ekelund et al., 2008). We report a trizygotic triplet pregnancy conceived by in vitro fertilization (IVF) in which triamniotic dichorionic placentation was identified in addition to hematopoietic chimerism in the monochorionic pair. A 29-year-old Caucasian, insulin-dependent diabetic woman gravida 2, para 1, conceived a triplet gestation with the assistance of IVF. Three embryos were transferred. An ultrasound performed at 7 weeks revealed three viable embryos in three gestational sacs. A thin membrane, with a direct, perpendicular insertion into a single placenta, was visualized between sacs B and C. There was no evidence of a lambda sign at the placental insertion site. These findings indicated monochorionicity between the B and C sacs. Fetus A was surrounded by a single amnion and a single chorion. The sonographic findings indicated a triamniotic, dichorionic triplet pregnancy. Genotypes of the B and C embryos were assumed to be identical due to the observed monochorionicity. At 12 weeks and 5 days, the ultrasound findings were confirmed. As part of a Down syndrome risk assessment, nuchal translucency measurements were 1.9, 1.3 and 2.2 mm, for triplets A, B and C, respectively. Ultrasound at 18 weeks revealed a gender discrepancy between the monochorionic fetuses, with triplet B demonstrating normal appearing female external genitalia and triplet C demonstrating normal appearing male external genitalia. Fetus A was noted to be female. Due to the gender discrepancy, amniocentesis was performed on all three gestational sacs. Separate needle insertions were performed for each fetus. Following withdrawal