Superfetation happens when twins are conceived many days or weeks apart from two separate ovulations and from two different fathers or the same father, the second one after the first has already implanted. Superfecundation are twins with different fathers but ovulations the same day.Superfetation happens when twins are conceived weeks apart from ovulation happening on different days. For superfetation to happen, two things have to happen:
1. You have to ovulate twice (dizyogtic) several weeks apart
2. You did get pregnant again after the first fetus has already implanted
The result of two pregnancies happening weeks apart is that the two twins are in different developmental stages and one fetus is more advanced in pregnancy then the other. This is an extremely rare event in humans and has been reported only a few times. Superfetation can happen with the same father or different fathers.
Superfecundation happens when twins are conceived by different fathers, usually within a couple of days from having sex with different men but from ovulation happening on the same day or within a few days.