You don't. I worked with a lady who swore up and down she couldn't get pregnant because of PCOS. She did have a couple miscarriages years ago so her and her husband stopped trying. Well everyone at work noticed her midsection kept growing. And 4 months before she went into labor I kept joking with her she was pregnant. She kept insisting she was just getting fat. But I told her she was going to end up on I Didn't Know I was Pregnant. Fast forward to February I walk into work and she is running out. Didn't even respond to my "Good morning". Then her husband calls work and tells us that she is in labor. I guess not even 20 minutes after leaving work that morning her contractions started. She wasn't even as fat as 400 pounds. Probably pushing 260-280. I'm surprised her baby came out healthy. She had bragged at work about getting wasted on Christmas, New Years, probably multiple other times and three days before she went into labor her, myself and some other coworkers went out for drinks. So not only was she obese, but drank the whole pregnancy too, never went to a doctor, and the baby is still healthy! Thankfully she is taking care of the kid and feeding him properly making sure he doesn't get fat like his parents. Hopefully he picks up the habits while he's still young and sticks with it.