Final answer:
Conjoined twins result from a single zygote that splits incompletely during early development, typically after the end of the first week post-fertilization, which is beyond the optimal period for the formation of distinct monozygotic twins.
Step-by-step explanation:
When considering the development of conjoined twins, it is essential to understand the process that leads to their formation. Conjoined twins result from the incomplete splitting of a single zygote during the early stages of development. Although a zygote can split as early as the two-cell stage, splits that lead to conjoined twins typically occur later than splits leading to separate monozygotic twins. Specifically, conjoined twins are believed to result when the splitting occurs sometime after the establishment of the primitive streak, which is usually around the end of the first week of embryonic development.
During the normal process of twinning, the division of the zygote to form two distinct embryos would ideally occur within the first 8 to 12 days post-fertilization. When the division is complete by 8 days, each resulting twin generally has its own amniotic sac and placenta. However, when the split occurs after the formation of the primitive streak, the embryos do not separate fully, which can result in twins that are conjoined.