A new study involving more than 1.1 million women worldwide has identified those most at risk for developing postpartum depression. They include first-time mothers, mothers under 25, and mothers of twins. The study also found that mothers over 40 with twins are the highest risk group of all. “The size of this study, in over 1 million new mothers, make the findings highly significant and definitive,” said senior author Dr. Jennifer Payne, who directs reproductive psychiatry research at the University of Virginia’s medical school.