The chances of your pre-pregnancybras continuing to fit during pregnancy and postpartum are essentially zero, so you might as well get some good bras — and if you're planning to breastfeed, there's no reason not to buy nursing bras.
First, buy a well - fittingbra: Once your pre-pregnancybra feels tight and uncomfortable (and is already on the loosest setting) it is time to buy a maternity bra.