Rebecca Roberts, 39, of Wiltshire, England, was already pregnant when a sonogram revealed that she had become pregnant again, three weeks after the first conception. The phenomenon, called superfetation, is incredibly rare; at the time of a 2008 study, there had been only 10 cases ever reported. The babies were delivered prematurely and, after a stay in the neonatal intensive care unit, are now healthy and happy at home. From The Hill:
Typically when someone becomes pregnant, the body undergoes hormonal changes that impede it from conceiving again at that time.
"Instead of stopping ovulation, she released another egg about three or four weeks after the first one, and the egg somehow miraculously managed to fertilize and implant in her uterus," Walker said.
While Roberts had been undergoing fertility treatments to help her conceive, drugs which are known to increase a person's chances of conceiving multiples at one time, Walker isn't convinced that that is the cause of her superfetation, and the rarity of the condition means little is known about its exact causes instance to instance.
See more baby photos at Roberts.Supertwins on Instagram.