Timing of Zygotic Division in Conjoined Twins
Conjoined twins result from zygotic division occurring more than 13 days after fertilization (answer b). 1
Embryologic Basis
The timing of embryonic cleavage after fertilization determines the type of twinning that occurs in monozygotic pregnancies:
0-3 days (or within 4 days): Division of the morula results in dichorionic-diamniotic twins with two separate placentas 1
4-8 days: Division of the blastocyst produces monochorionic-diamniotic twins with a single shared placenta and two amniotic sacs 1
8-12 (or 8-13) days: Division results in monochorionic-monoamniotic twins sharing both placenta and amniotic cavity 1
Beyond 12-13 days: Division occurring after this critical threshold results in conjoined twins 1
Key Pathophysiologic Concept
Conjoined twinning occurs specifically when the division event happens at approximately the primitive streak stage of development, around 13-14 days post-fertilization. 2 This late division is exclusively associated with monoamniotic monochorionic placentation. 2
The mechanism involves incomplete splitting of the embryonic axis rather than fusion of previously separated twins. 2, 3 This incomplete division at such a late developmental stage prevents the formation of two separate individuals, resulting in twins that share a body with varying degrees of organ duplication and fusion.
Clinical Significance
The case described—two fetal heads arising from a shared body—represents a form of conjoined twinning (likely dicephalus variant). 3 This confirms that division must have occurred more than 13 days after fertilization, making option b (More than 13 days) the correct answer. 1, 2
Conjoined twins are extremely rare, occurring in approximately 1 in 50,000-200,000 births, with about 60% being stillborn. 4 The prognosis depends heavily on the extent of organ sharing, particularly cardiac structures. 4