When is the latest time a singleton pregnancy can become a twin pregnancy?

Single embryo splitting into twins refers to the process in which a single fertilized egg divides into two embryos. There is generally no exact time for this division, but it usually occurs no later than 13 days after fertilization. The details are as follows:

Embryonic splitting refers to the division of a single fertilized egg into two embryos during the early stages of embryonic development, which typically happens within 13 days after fertilization. If both embryos develop successfully, they will form twins. Depending on the timing, this can be classified into four categories: splitting within 3 days after fertilization, splitting between 3–8 days after fertilization, splitting between 9–13 days after fertilization, and splitting after day 13. Generally, the survival rate of the fetuses is higher when splitting occurs within 3 days, between 3–8 days, or between 9–13 days after fertilization. However, if splitting occurs after day 13, not only is the survival rate lower, but also by this time the embryo has already developed clear morphology and structure, making embryo splitting or merging less likely.

In summary, the latest time for a fertilized egg to split into two embryos is generally 13 days after fertilization. However, note that the timing may vary depending on individual circumstances. Therefore, if there is concern about such a situation, it is recommended to promptly visit a hospital for appropriate examinations.