During pregnancy and after birth breasts are much bigger and require appropriate care. They are also a heavy load, to which your spine isn't used to. Choosing the right bra is very important for the health of your back, and it also prevents stretch marks and saggy breasts.
It's best to buy the first nursing bra already during pregnancy. Choose the band size according to your size before pregnancy. If you have put on a lot of weight, pick one band size bigger than before pregnancy. When it comes to choosing the cup size, pick one or two cups bigger than before pregnancy. In the beginning it's best to choose a soft cotton bra, which is comfortable for lying and sitting, instead of an underwired bra. Right after birth, during hyperlactation, your breasts will be swollen, but later they will shrink a bit.
When I tried on bras in various shops, it turned out that the sizing was completely different. I noticed that in one shop size 90C was equal to size 80E in another shop. Therefore, always try the bra on before buying, instead of trusting the package. Before hitting the shops, take the measurement under your bust. Do it while exhaling. Subtract 5 cm from the measurement, then round it down (if you measure 92 cm under your bust, subtract 5 cm - 87 cm - and round it down to 85). When your band size is appropriately chosen, the bra shouldn't slip even if the straps slide down. The cup size should be chosen in the shop. When you try the bra on, bend down and fit both breasts in the cups so that they fill the cup from the sides and the bottom (gather the breast with your palm from under the arm and breasts). The breast shouldn't stick out from under the wire or elastic band. Regulate the straps so that they don't dig in, but still hold the bra in position for a longer time. Check that the bra doesn't chafe the skin at the armpits. If it does, the cup size is too small.