Cup size is typically measured as the difference between your high bust and your full bust.
A = 1″ difference, B = 2″, C = 3″, D = 4″, E = 5″, F+ = 6″ difference or more.
Your cup size selected for sewing is likely to be similar to your bra size though it may not be the same for several reasons. Bra cup size changes in relation to band size (a 34C bra should accommodate the same size breasts as a 36B bra) so a “B cup” is not a fixed amount. “B cup” breasts on a size 2 frame are much smaller than “B cup” breasts on a size 18 frame. A bra size accounts for your under bust measurement which is more descriptive of your rib cage shape. A sewing cup size accounts for your high bust measurement which is more descriptive of your frame size.
Because each person’s body has its own unique shape, for perfect fit you may still need to adjust a pattern even if it is the “correct” cup size to start. For example, you may measure as a D cup because you have a 4″ difference between high bust and full bust but you may find that actually get a better fit with a C cup because your ribcage is shaped differently than the pattern’s block. However, selecting a pattern based on cup size can still make fitting much easier. With a 4″ difference between high bust and full bust, you are likely to need much smaller adjustments when starting with a D cup pattern than an A cup pattern.
During pregnancy, your breasts are likely to grow and you will probably find yourself 1 to 3 cup sizes larger than before pregnancy, depending on how much weight you gain and how your own unique body changes with weight gain and hormones. Maternity patterns are often made on a block with a larger cup size than a non-maternity block. This is important to remember when selecting a “maternity-friendly” pattern (i.e. a pattern that was written for normal wear but that may be worn during pregnancy because of its cut). MaternitySewing allows you to filter patterns by cup size while browsing maternity and nursing sewing patterns so that you can easily find a pattern that is drafted for proportions similar to your own.
Many women find that their bust stays larger while nursing and that they lose pregnancy weight from other areas of their body before losing from their bust. Therefore you may need to select nursing clothing in your pre-pregnancy size but with a larger cup size. Again, these are generalizations that may not be true for your unique body so make sure you use your honest measurements for size and cup size selection.