Twin Pregnancy with Differing Gestational Ages: Possible Explanations

It is highly unlikely that a woman can carry two babies with a one-week difference in gestation. It is more likely that the gestational age of one of the babies was miscalculated or that the woman is experiencing a condition called heterotopic pregnancy, where one baby grows in the uterus and the other in a fallopian tube. This condition is rare, but it is possible.