As girls reach puberty, they closely monitor the amazing and mystifying ways their bodies shift toward adulthood. Of all the changes, though, none is more closely watched and wondered about than breast changes. Girls watch the silhouettes of friends for hints, look to their moms and to movie stars and wonder: What will mine look like? How big will they be? When will I need a bra - and what size will it be?

For some women, their first bra wasn't really for support. Those training bras - simple, bandage-like pieces of fabric with basic clasps and, often, a tiny, delicate bow sewn between the cups - were probably bought to provide coverage during gym class. For others, that first bra was an absolute necessity for support and coverage.

No matter what her size, a woman's breasts need support. That's because a woman's breasts have no natural support system. The breasts contain no muscles to hold them up, instead relying on the skin and a network of underlying connective tissue called the "Cooper's ligaments" for minimal support.

As a woman moves, so do her breasts - vertically, horizontally, and in an overall figure-8 like motion. If not properly supported, breasts can move too much and cause pain and discomfort - especially during exercise. A bra can reduce excessive motion and prop up the breasts to help slow sagging that comes with age and movement.

Unfortunately, it's estimated that 8 out of 10 women wear the wrong size bra. A woman chooses a bra size based on two components - the cup size, noted by letters, and the band size, noted in inches. Cup size is measured at the fullest point of the breasts, and the band size is determined by measuring the distance around a woman's back right beneath her breasts.

There are lots of reasons why women calculate this bra equation incorrectly, like not switching sizes when weight fluctuates. In many cases, women pay more attention to cup size than to band size, often choosing a cup size that is too small and a back size that is too large. A lot of cultural emphasis is put on cup size. Stuck in an A-cup, a woman may feel less feminine, while going beyond D cup can seem like crossing into forbidden "fat" territory.

But when a woman wears a bra in which the band size is too large, she depends on tightening the shoulder straps to keep the breasts propped up. Ill-fitted shoulder straps droop or dig into the skin and can cause muscle tightness that may trigger aches and pains. A properly-fitted band fits around the smallest part of a woman's back, midway between shoulders and elbows. At this position, a bra can provide adequate support, lift the breasts, and accentuate a woman's waistline. Consider going up a cup size and down a band inch.

So, forget the stigmas and go for fit. In fact, go for a fitting! A professional bra-fitting can help women find not only the perfect bra size but also the perfect bra type for her figure. A full-cup bra will cover much of the breast, while a balconette (A.K.A. shelf bra, demi-cup, half-cup) is cut horizontally across the breast. A plunge-style bra dips down into the cleavage.

Women should also invest in a well-fitted sports bra. All the up-and-down, side-to-side, and figure-8 movement during exercise can be a real pain, no matter what size of breasts a woman has. Luckily, the sports bra has come a long way since its beginnings as sewn-together jockstraps. Nowadays, a woman can choose the more traditional "compression" sports bra, which flattens the breasts against the chest. Or she can opt for a sports bra with moulded cups to support each breast separately. The compression sports bra is thought to be more effective to reduce movement for smaller breasts, while the moulded cup sports bra is thought to be more effective for larger breasts. However, some research has noted that these double-cupped sports bras more effectively reduce motion for both larger and smaller breasts. Women should try both types to see what is most comfortable for them.

Amy Toffelmire