The best time to buy your nursing bras is any time that suits you during the last month of your pregnancy. You should buy 1 cup size larger again from your pregnancy size (eg from D - E), but the good news is that you need to choose your pre-pregnancy rib size as your ribcage will start to shrink back (eg from 36 to 34) after the birth of your baby. If you buy any larger in your rib size, you will lose your breast support.
In other words, if you were size 34C before your pregnancy, you will have been 36D during your pregnancy, and you will therefore need size 34E for breastfeeding your baby.
Around three or four days after the birth of your baby, your milk will arrive and you will feel huge! After that time you should fit into your breastfeeding bras - don't expect them to properly fit you before then.
We recommend that you buy two nursing bras to take into hospital: one with good support for day (Underwired Nursing Bras are ideal) and a softer one for breastfeeding at night and sleeping in. Once you are confident that they are the perfect fit, you can buy more.
Around 12 - 16 weeks after the birth, you will hopefully be in a good breastfeeding routine and your breasts will begin to feel less full and not so heavy.