Mood changes during and after pregnancy are surprisingly common. But despite that, it's not always easy to recognize what's going on. Indeed, fitness guru Jillian Michaels reveals in a new interview that it was a challenge for her to understand what was happening when her partner, Heidi Rhoades, was suffering from postpartum depression (PPD) after the birth of the pair's son.“While everybody goes, ‘Oh yeah, postpartum, I’m so scared of it,’ I don’t think they realize that it can come in so many different shapes and forms,” Michaels tells People . But, after giving birth to their son, Phoenix, Michaels' says Rhoades felt isolated and lonely. “I didn’t know what was wrong, I didn’t know how to help her," explains Michaels, who just released a new book about pregnancy According to the American Psychological Association , about one out in seven women will experience PPD. And, although having dealt with depression before raises your risk for PPD, it's the first episode of depression for about half of those women.If you think you or your partner may be suffering from PPD, the most important thing is to seek professional help — this is not the kind of thing to try to handle on your own. Treatment usually consists of therapy with or without an antidepressant medication.But it's also crucial to be able to speak honestly with those around you about what's happening, which isn't always easy in a culture that expects new parents to be constantly overjoyed. So it's great to see Michaels and Rhoades bringing their story into the light.