Twins that share the same chorion but with separate amniotic sacs
Twins may be classified according to the degree of separation in utero, i.e. according to chorionicity (placentae) and amniocity (amniotic sac): (1) dichorionic-diamniotic twins, (2) monochrorionic-diamniotic twins, (3) monochorionic-monoamniotic twins, and (4) conjoined twins. In monochorionic-diamniotic (MoDi) twins, the twins share the same placenta and (mostly) are diamniotic, i.e. having two amniotic sacs. This is common in monozygotic twins (i.e. 60-70%).1 MoDi twinning occurs on day 4 to day 8 after fertilization.2 It occurs in 60% to 70% of the pregnancies with monozygotic twins3, and in 0.3% of all pregnancies.4 Thus, most of the MoDi twins are monozygotic twins.
- dichorionic-diamniotic twins
- monochorionic-monoamniotic twins
- conjoined twins
- amniotic sac
1 Curran, Mark (2005-11-02). “Twinning”. Focus Information Technology. Retrieved 2008-10-10.
2 Shulman, L. S. and Vugt, J. M. G. van (2006). Prenatal medicine. Washington, DC: Taylor & Francis. pp. 447.
3Curran, M. (2005-11-02). “Twinning”. Focus Information Technology.
4 Cordero, L., Franco, A., Joy, S. D., & O’shaughnessy, R. W. (December 2005). “Monochorionic diamniotic infants without twin-to-twin transfusion syndrome”. J Perinatol25 (12): 753–8.