Determining zygosity in early pregnancy by ultrasound.

OBJECTIVES

First-trimester ultrasound can reliably determine chorionicity but not zygosity. We set out to investigate whether it may be possible to determine zygosity using ultrasound by noting the number of corpora lutea (CLs), structures which reflect ovulation. In the presence of a dichorionic twin pregnancy, the identification of one CL would suggest that twins are monozygotic whereas two CLs implies dizygosity.

METHODS

This was a retrospective analysis of predominantly spontaneous twin pregnancies presenting for an early pregnancy ultrasound at 5-8-completed weeks of gestation. Placentation was correlated with presumed zygosity as predicted by the number of CLs present.

RESULTS

Of 33 twin gestations, chorionicity was compatible in all cases with the predicted zygosity. In 15 cases one CL was seen and these were designated monozygotic. Of these, four were of monochorionic placentation and 11 dichorionic. The remaining 18 cases had two CLs and were presumed dizygotic; all were of dichorionic placentation.

CONCLUSION

We propose a novel technique of zygosity determination during very early pregnancy which may have implications both clinically and in genetic research involving twins. However, this study requires further verification by comparing ultrasound results with DNA evidence taken after birth.