Bra extenders are a great way to get a longer life out of bras that have become too tight through the band. You get them as 1, 2, 3 or 4 hook options to fit different bras. They usually come in black, white and beige to blend in with the most common bra colours. To use them simply attach it to one end of the back fastening on your bra then put it on and do it up as normal.
Bra extenders are cheap and easy to find in most lingerie stores, they are often in sewing supply stores and are sold online on many sites including Amazon. Sometimes the spacing between the hooks in a row can be different, see the two different 2 hook options in the top image for an example. If you have a particular bra in mind then bring it with you when you are buying the extender, that way you can check they will definitely line up correctly.
During pregnancy most women will become wider through the chest, extenders are a great way to get more use out of your current bras before you have to switch to a maternity bra.