Most department stores have fitters who are professionally trained to assist you. The problem is they are often busy so one should make an appointment for this service.
If this is not available, you can measure yourself. This gives you a starting point in finding the proper size.
First, measure under the bustline and add five inches to this measurement. Then measure across the fullest part of your breasts. You should be wearing your best fitting bra while taking this measurement. The difference between these measurements gives you the cup size. If for instance your first measurement is 34 (29 + 5) and the second measurement 36, you would take a 34 B. Each inch difference equals a cup size. 1 = A, 2 = B, 3 = C, 4 = D, 5 = DD, etc. If you are already starting out at a 42 inch or greater measurement around the bustline, you do not need to add the five inches. Start with the measurement itself for the band size.
Remember, this is only a place to begin. There are anatomical differences among women so this is not an exacting science. Also, manufacturers do not size their products exactly the same. Even within a specific brand, all styles fit differently. Therefore, you may wear a 38 DD in one style, and a 40 D in another. You may have to try on several bras to find the best fit for your body.
One common mistake is to increase the band size to compensate for too shallow a cup size. The band should fit snug around the bustline, but not uncomfortably. All of the breast tissue should fill the cup without any bulges above the fabric. Underarm tissue should also be contained and again, not bulging above the fabric. During pregnancy, it is very important to have a well supporting bra.
If you have always worn an underwire, you don't necessarily have to give it up. The rules for underwires remain the same pregnant or not, you should not feel any pokes when it is on. Often, people say underwires are uncomfortable. If you are uncomfortable, it most likely means you are not in the right bra for your body type. A professional can guide you to the proper fitting bra. Again, you should not feel the wire in any way. It is only there to add support to the garment. During pregnancy, if you are carrying the baby unusually high, even those who have always worn an underwire may find it to be uncomfortable and should switch to a soft cup style.
Nursing bras are designed for the convenience of nursing and are not suggested to be worn during pregnancy as most cannot give the support needed for enlarging breasts. It is a good idea to hold off until two weeks prior to your due date (or earlier if you are having complications) before purchasing a nursing bra. It is suggested that you buy two sizes, the largest size that you are during pregnancy and one cup size larger. Since it is impossible to know what size you will be, you will have the option of the two and can exchange the one that does not fit. When you are established with your milk supply, you may find a difference again in your breast size and you should re-evaluate whether your current bra still fits properly.
During pregnancy, you want to do whatever you can to look and feel better. A well fitting bra can give you the support you need as well as the confidence in knowing that you look good.