In 2009, Julia Grovenburg of Fort Smith, Arkansas, got pregnant twice -- two weeks apart -- and ended up delivering two healthy babies by Caesarean section in December of that year. But the babies were not twins. The phenomenon is called superfetation, a rare type of pregnancy in which an embryo is formed from a separate estrous cycle after another embryo, or fetus, is already present in the uterus.
These pregnancies occur when a woman continues to ovulate after becoming pregnant, and a second fertilized egg is able to implant itself in the lining of the uterus. Typically, hormonal changes prevent continued ovulation and a thickening uterus usually prevents a second embryo from attaching.
A rare way to get pregnant: