TRENTON, N.J. — The U.S. Food and Drug Administration has approved the first drug specifically developed for severe depression after childbirth. The agency on Tuesday approved Sage Therapeutics’ Zulresso (ZUHL’-ress-oh), an IV drug given over 2 ½ days. In a company-funded study of new mothers with moderate or severe postpartum depression, half the women given Zulresso had depression end within 2 ½ days, about double the rate of those in a comparison group given dummy treatments. Postpartum depression affects about 400,000 American women a year. It’s often treated with antidepressants, though they can take several weeks to help and don’t always work. Sage said Zulresso will cost$34,000without insurance, plus costs for the stay in a hospital or infusion center.