Most bra manufacturers have specific instructions for how to measure when fitting their bras. For this reason it's important to measure according to the manufacturer's instruction. Generally, band size (number) is determined by measuring around the body either directly under the arms or under the breasts and straight around the body holding the tape so that it is straight across the back. Whether or not you measure under the arms or under the breasts can vary the measurement greatly, so be sure to know how your bra's maker suggests measuring. Generally, an odd number band measurement is rounded up to the next even number; for example a 37 inch band measurement would be rounded up to a 38. Cup size (letter) is usually determined by measuring around the fullest part of the bust, while wearing a good-fitting bra, and around the body holding the tape so that it is straight across the back. The difference between the band measurement and the cup measurement in inches determines the cup size. For example, a 3 inch difference between the band measurement and the bust measurement would indicate a C cup size, a 4 inch difference would indicate a D, etc.

It's best to measure for your nursing bras sometime during the last month of your pregnancy. Choose a bra that fits well and comfortably but that allows for some extra room in the cup. This will accommodate continuing breast enlargement during the last few weeks of pregnancy as well as the early days/weeks postpartum when the breasts swell with milk. Purchase only one or two bras at this time because most likely your size will change after birth. Around the end of the second week, your breasts will not be swollen anymore and milk production will have begun to regulate. At this time, re-evaluate your size and purchase as many bras as you think you will need to always have a clean one on hand.

A bra that fits well does not pinch or bind the breasts in any way. There is good coverage in the front and on the sides so that cleavage does not spill out in either area. The band sits straight across the back and does not ride up the back. The strap sits comfortably on the shoulder, not digging into the skin or slipping off the shoulder with movement. A bra that's fit is too small can put you at risk for less-than-optimal milk flow and plugged ducts/breast infections. A bra that's fit is too large will not provide enough support and may cause discomfort either in the breasts themselves, in the back, or both. The breast tissue should be lifted and supported if the bra fits well.

Many mothers have been told that underwire bras should not be worn while nursing. This is optimal but not always feasible for all mothers. Some mothers have worn underwire bras before becoming pregnant and feel comfortable in nothing less. Others, especially larger-busted moms, may require the extra support and lift that an underwire provides in order to achieve a good fit.

Underwire bras require good fitting even more so than softcup bras. An underwire bra that is too small will dig into, pinch, and bind the breast tissue, making plugged ducts and breast infections more likely. Mothers should never sleep in an underwire bra as it is more likely that the wires will compress the breast tissue with lying down for prolonged periods of time. If plugged ducts and/or breast infections are recurrent, the size of the underwire bra should be re-evaluated or it should be replaced with a softcup bra until weaning occurs. Generally, however, an underwire bra can be worn during the daytime as long as it fits well and the mother is not experiencing repeat episodes of plugged ducts or breast infection.