When girls begin puberty, usually between the ages of 8 and 13, their breasts begin to develop starting with just a little swelling under the nipple. This is known as breast budding. You may notice that one of your breasts starts developing before the other or that one is growing more quickly. Many times this difference in breast size evens out once a girl is older, usually around age 20. But it's also perfectly normal for women's breasts to remain different sizes even when they're fully developed.
Some girls with different-sized breasts worry that there's something medically wrong, but chances are, for a teen, there isn't. If you're really worried about your breast size, talk to your doctor or gynecologist. He or she should be able to reassure you that you don't have any health problems. In fact, the good news about noticing your breasts are different sizes is that it means you're aware of your breasts - and being aware of your breasts can help you stay healthy! Learning how your breasts normally look (and feel) can help you notice any changes that might not be normal. Your doctor can teach you how to do a breast self-exam that will help you become familiar with your breasts so you can identify any changes.
Some girls worry about their different-sized breasts for cosmetic reasons. Many girls with different-sized breasts or nipples just accept that they are normal and don't do anything differently. But other girls feel self-conscious. It's likely that no one else notices your breasts look different, though. Other people usually can't see the difference in the size of a girl's breasts - even if that difference is a full cup size.
The fact is that exercises, supplements, or diets won't change the size or shape of a girl's breasts, only plastic surgery can do that. (And most doctors recommend that a girl wait until her breasts have finished growing before getting plastic surgery.) Some girls decide to wear especially supportive bras or special inserts that make their breasts appear more equal in size. Talk to a nurse or lingerie salesperson if you want some suggestions for evening things out. It may also help to talk to your mom, older sister, or an aunt or grandmother - they may have worried about having different-sized breasts during their teen years, too.
Everyone develops differently, and girls aren't the only ones with questions. A guy may wonder if his penis is developing normally, too. We humans aren't as evenly proportioned as we appear at first glance!