Which type of twins results from splitting after day 12 of fertilization?

The type of twins that results from splitting after day 12 of fertilization is conjoined twins. This occurs when the embryo, which initially develops as a single entity, does not fully separate into two individual embryos. If the splitting occurs late, typically between days 13 to 15 post-fertilization, the cells can divide partially, leading to conjoined twins who share certain organs or body parts.

Conjoined twins arise from one fertilized ovum, which is not completely divided into two separate individuals. Because the split happens later than the 12-day mark, the twins remain physically connected.

In contrast, twins that separate earlier in the gestation process generally fall into different classifications. Di/di twins emerge from separate fertilized eggs and will each have their own placenta and amniotic sac. Mo/di twins occur when a single fertilized egg splits before day 4, leading to two embryos that share the same placenta but have separate amniotic sacs. Mo/mo twins result from a cleavage between days 4 and 8, where the embryos share both the placenta and the amniotic sac but remain separate individuals. Each of these options represents a scenario where complete division occurs, unlike with conjoined twins.