title: Two corpora lutea seen at 6-13 weeks' gestation infers dizygosity among spontaneous same-sexed dichorionic twins.
abstract: Current ultrasound techniques can accurately determine the chorionicity of twins, but not zygosity. We previously proposed that the zygosity of spontaneously conceived twins can be determined at early ultrasound, where 2 corpora lutea infers dizygosity, and 1 implies monozygosity. Here we did a case series, comparing zygosity predicted using this method with definitive DNA genotyping of twins after birth. We retrospectively identified 14 ultrasound reports of spontaneous twin pregnancies at 6(+0 days) to 13+6 weeks' gestation, where both ovaries were seen and the number of corpora lutea documented. We visited all twin pairs, obtained buccal smears, and determined zygosity by genotyping 9 independent microsatellite markers. All 8 cases where 2 corpora lutea were seen were dizygotic pregnancies. One further case where 3 corpora lutea were seen was also dizygotic. All 3 sets of monozygotic twins had 1 corpus luteum. There were 2 cases incorrectly assigned, where 1 corpus luteum was seen in dizygotic pregnancies. We conclude if 2 corpora lutea are seen at a first trimester ultrasound of spontaneously conceived dichorionic twins, they appear to be almost certainly dizygotic. However, if 1 corpus luteum is seen in dichorionic twins, zygosity cannot be determined with certainty since it is either monozygotic, or dizygotic where a second corpus luteum has been missed.
authors.affiliation: Southern Health, Women's and Children's Program, Victoria, Australia.
authors.forename: Jean
authors.initials: J
authors.lastname: Woo
authors.forename: Stephen
authors.initials: S
authors.lastname: Tong
authors.forename: Megan J
authors.initials: MJ
authors.lastname: Campbell
authors.forename: Leanne
authors.initials: L
authors.lastname: Wallace
authors.forename: Simon
authors.initials: S
authors.lastname: Meagher
authors.forename: Grant W
authors.initials: GW
authors.lastname: Montgomery
authors.forename: Fay
authors.initials: F
authors.lastname: Chao
authors.forename: Weng
authors.initials: W
authors.lastname: Chan
authors.forename: Beverley
authors.initials: B
authors.lastname: Vollenhoven