Superfetation (also spelt superfoetation, based on a hypercorrection — see fetus) is the formation of a fetus while another fetus is already present in the uterus. When there are two separate instances of fertilisation during the same cycle, rather than different cycles, it is known as superfecundation.
Superfetation is claimed to be common in some species of animals, but is extremely rare in humans. It can occur only where there are two uteri, or where the menstrual cycle continues through pregnancy.
Reports of superfetation occurring long after the first impregnation have often been treated with suspicion and some have been clearly discredited. Other explanations have been given (and demonstrated) for different levels of development between twins. Artificially induced superfetation has, however, been demonstrated although only up to a short period after insemination.
once you go into labor though you have to have both babies if this actually happens which is super rare and there have only been 2 actual known cases an happend in the UK. both babies would be delivered at the same time due to the body producing the hormone that causes labor and the water being broken you cant have a baby later, there are allot of people that beleive that you cant get pregnant while breast feeding and probably ended up pregnant right after the birth of the other child is a more rational explanation for this.