Teenage girls might be excited about getting their first bra, event to the extent that they buy one only because it looks nice. However, the one thing that will make their boobs look good is the right bra fit.
Up to 70% of women don’t wear the right size bra, leaving them with breast pain, back pain, or simply an ugly fit that won’t do them any justice.
Getting it right:
You can bring your daughter out to have her breast size measured. Some shops even do it for free, just ask whether or not the service is available.
Those who would rather do it themselves will need a measuring tape.
To find out chest size, measure directly under the boobs. Pull the tape flat on the skin around the back and right under the breasts. Take the measurement in inches.
To measure breast size, pull the tape flat around the back and the fullest part of the breast, and then measure in inches.
Buying bras:
Those who have measured themselves correctly and find that the bra still doesn't fit need to adjust the straps before trying a different size.
Make sure to have a good sports bra or snug tank top with a comfort underneath. They are important for protection against pain and sagging in the future.
Remember that bigger breasts need wide, strong straps to support them.
Always remind your daughter that they will look sexiest when they feel confident and comfortable.
How do I know if my bra doesn’t fit?
If a bra doesn’t fit smoothly over the cup size, it may be an indication that the bra is too big.
When boobs are popping out on the side or over the cup, then the bra is too small.
The bra should never dig into the skin anywhere. If your daughter has red marks under her breasts or from the shoulder straps it may mean that she needs a new bra.
When the back strap slides up the back, then going down a back size will be the solution.  
Remind your daughter that her breasts will be a different size in different stages of her life. During the teen years, breast sizes change due to hormonal changes or being on the pill. She could make a habit of regularly checking her breast size just to be sure.