Superfetation is an event when the second pregnancy occurs during the initial one. In other words, a new fetus is being formed while another one is already present in the womb. Even though superfetation is considered common in some animal species, it is extremely rare in humans. So far, only a few cases of human superfetation have been reported. 

Several days ago, the surprising news started to circulate on the internet. Rebecca Roberts gave birth to twins that were conceived 3 (three) weeks apart. As CNN reports, "baby Noah was joined in utero by fraternal twin Rosalie some three weeks after Noah's conception."  This is the latest example among other 14 cases of superfetation that have been revealed worldwide. 

It is beleived that there is a link between superfetation and fertility treatment.