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.