Yug Ylimaf - S11-E4
Corrected entry: Bonnie had Susie in season 7. In this episode they go back in time before Susie was born, Stewie is a year older than Susie so it makes no sense why he would be re born again. He's always a year old throughout the entire series.
Correction: They didn't go back in time. When the time machine broke, time started moving backwards. When Stewie saw Bonnie was pregnant, he realised events were being undone, which means he would soon be unborn. Time kept moving backwards until Stewie was unborn. When Brian fixed the machine, time went forward and Stewie was born (which would be a year before Susie was born).
Bishop73