Sometimes a woman produces more than one egg at a time. If two eggs are fertilised by different sperm, then she conceives non-identical twins (also known as fraternal or dizygotic twins). The babies are no more alike or non-alike than any other brothers or sisters. Each twin has its own placenta and its own bag of water (amniotic sac) and for this reason they are described as dichorionic diamniotic twins (two placentas and two sacs). It may look on the scan as if the twins share one placenta, but in fact the two placentas remain separate even if they have grown to lie closely together.