In general maternity bras are those designed to be worn throughout pregnancy. They tend to feature non-wired cups (as underwires can damage the rapidly developing breast tissue), extra hooks on the back to allow for expansion of the ribcage and fabrics designed for firm support, but with a bit of stretch to allow room to grow.
You can start to wear maternity bras at any stage of pregnancy but they are recommended as soon as your current bras start to feel at all restrictive, which in general tends to be before the 4th month of pregnancy. I found that my bust was the first visible sign of my pregnancy, well before my baby bump started to show, and this is often the case. You may also notice breast tenderness in early pregnancy and again this is a sign of the pregnancy hormones starting the growth of the milk producing tissue, in readiness for breastfeeding.
Nursing bras share many features with maternity bras, with the main difference being the drop cups on the nursing bras for easy breastfeeding access. Whilst it is perfectly OK to wear nursing bras though pregnancy (and if this is a second child then you may still be breastfeeding), the size that fits though pregnancy may well be too small once your milk "comes in", around 3-4 days after the birth. However, you will gradually get smaller again, and I found that once my son was over a year and eating a fair amount of solid foods as well as breastfeeding, then my cup size dropped.
For breastfeeding you need easy access to the breast, often you will have baby in one arm, so features such a drop cups that can be opened and closed with one hand are ideal. Whilst there are other options, such a zips, available, these tend to be more fiddly. Standard drop cups have a retaining strap at the side that keeps the shoulder strap in place so that you can close the cups easily after feeding. They allow for full breast access, which can be helpful in the early days as you are learning this new skill!
"A" frame cups have more material inside the cup that, when opened, provides a supportive sling for the breast. These can be particularly helpful for mums with larger busts although you need to make sure the inner fabric isn't restrictive.
As the bust is developing throughout the pregnancy, the best time to be fitted for your nursing bras is towards the end, around 37 weeks (so 2-3 weeks before your baby is due). By this stage the majority of the physical breast development is complete (you may start noticing that you are leaking a few drops of colostrum - the milk that is designed perfectly for a newborn's first few days) and you may be feeling your ribcage ease as your baby moves down into your pelvis, ready for birth. These both help to predict your nursing bra size more accurately.
Whilst leaving buying your nursing bras till after your baby is born may seem like a good idea, don't forget that getting out of the house with a new baby and all the related paraphernalia can be a challenge and that nursing bras will make breastfeeding easier, especially in those first few days.
