Q&A: Why do our stomachs get thinner and our hips get wider during puberty? And how long after your first period does this happen?

Great question! First, let’s address the belly question. Often, before you go through puberty and start your period, your body will grow – a lot! Before you grow in height, sometimes your body will store some fat around your waist in preparation for growing taller. As you grow taller (while eating healthy and exercising) that fat gained around the waist goes away, kind of. (The fat cells are still there. They are actually just smaller.) Now for the hips – One fact is that your heredity will partly determine your height and your overall body shape. So, as you grow taller, your body starts producing all kinds of hormones, which are chemicals that tell parts of your body what to do. During that time of growth, your hips widen (depending partly on what your genes, aka heredity, told them to do!) This widening helps with having babies in your adult years. Interestingly, when your hips widen, your stomach will look smaller than it used to because of proportion.
There are a whole lot of changes going on during puberty, aren't there? It might be surprising to see your body changing in so many ways, but that's what it's designed to do. Hips get wider to prepare your body for the future so it's capable of carrying and delivering a baby. And as your hips widen, it may cause your stomach to get a little flatter (although it doesn't always). There's no set timetable for when all of this happens or how long it takes. Each person's puberty experience is different. But if you're worried that it's happening too quickly or too slowly, try to remember that everyone goes through it at the pace we're supposed to.
You know how as you get older you get taller? Well other parts of your body "grow up," too. If you are going through puberty (which happens gradually over several years) you may notice your breasts getting fuller and your hips/thighs getting wider. Some girls also gain a bit of weight during this time, too. All of this, along with your first period, are simply signs that you are getting your adult woman body. These changes can happen before or after your first period and will most likely happen so slowly you won't even notice for a while! Puberty is just like getting taller, everyone grows at different rates. So try not to compare your changing body to  others!