Most mothers are diagnosed with postpartum depression - the intense sadness, anxiety or despair that occurs within the first year after giving birth, according to the Centers for Disease Control and Prevention. It affects about one in nine women, although the rate may be as high as one in every five women, the CDC finds.

Up until this point, new mothers experiencing postpartum depression have been prescribed the same antidepressants used for treating depression in the general population, such as selective serotonin reuptake inhibitors. The drugs can take weeks to take effect, and do not address the hormonal changes that women go through during and after pregnancy.

But on Tuesday, the Food and Drug Administration approved the first drug specifically developed for postpartum depression, called brexanolone, or Zulresso.

Brexanolone is novel because it has a synthetic form of the hormone allopregnanolone, a progesterone derivative, in it. The hormone increases throughout a woman's pregnancy and then plummets after she gives birth, a possible contributor to postpartum depression.

"This can potentially transform women's lives and that of their families," said Dr. Steve Kanes, chief medical officer of Sage Therapeutics, the Cambridge, Mass., biopharmaceutical company that developed brexanolone. "It's not just the mother who suffers when there's postpartum depression. It's the newborn. It's the other people in their family."

Brexanolone is not a pill. The drug is delivered intravenously over the course of a 60-hour infusion, meaning it must be administered in a medically supervised setting, such as a skilled facility or a hospital, rather than at patients' homes.

Clinical trials for the drug were promising - not just in the number of women it helped, but in the near-instantaneous relief that is provided.

In double-blind, placebo-controlled trials, many women with moderate to severe postpartum depression saw a marked improvement of their symptoms within just 24 hours of receiving the drug. That improvement was still present 30 days after the infusion, the length of the trial.

The drug's approval comes just weeks after the FDA signed off on esketamine, a fast-acting nasal spray that uses the active ingredients in the club drug ketamine, as a treatment for severe depression.

The most common side effects during the brexanolone trial were drowsiness and dizziness. The drug is not believed to have any long-term safety concerns. Kanes, Sage Therapeutics' chief medical officer, said he expects it will be deemed safe for all mothers, including breastfeeding mothers, but the company is waiting for an FDA ruling on breastfeeding.