Though some women will find that their breasts start to change or get sore almost immediately in pregnancy, others may find they don't need to change their bra at all until they invest in a couple of feeding bras after the birth.
Most women will see some change as their breasts grow - in some cases by as much as two inches - which not only means usual bras feel tight and uncomfortable, but may well not be giving this newly increased weight the support it needs. The result can be an additional strain on your back, which is already being challenged by your growing bump and probably your changes to the way you move about.
In feeding bras, a cotton-based material is preferable to a synthetic material because excess milk can mean the bra gets a bit damp. In maternity bras, this isn't a problem, but cotton bras do mean you will feel a little less sticky when your increased blood flow raises your average body temperature.

When is a good time to change?
You probably don't need to change your bra until you are in about the fourth month of your pregnancy but be aware of your own needs in case changes demand a switch much sooner than that.

You might think that it's not worth investing in maternity bras if you're then going to have to buy feeding bras too, but your breasts will change shape. It is possible to buy some really nice maternity bras that have 'drop down' cups so you can use them for feeding too. If you do this, you need to make sure the bra has several back hook options as your width size will go back to a more normal size after birth. You should also be aware that some women find their cup size dramatically increases when their milk finally does come in (about three days after birth). However, this can vary and some women will not notice a vast difference.

Underwired or not underwired?
Although there is no concrete evidence to back it up, many health professionals advise against wearing underwired bras during pregnancy.
This is because underwired bras by their nature, are more restrictive underneath the breast, and could supress the growing milk ducts. Their tightness could also cause the increased blood flow to be restricted.
However, women with larger breasts who already feel the need for the support of underwired bras, will not want to switch to something with less support as their chest grows during pregnancy.

If your underwired bra is not too tight already - make sure it does not leave a red mark around your body when you take it off - you might be OK. Some underwired bras are suitable for maternitywear and if you get fitted at a reputable department store, ask to see someone who knows about maternity fittings as well as normal bra fittings, as they will be able to advise you further, depending on the ranges available. If you go for a fitting (these are usually free), wear a bra that you are most likely to wear day to day so the advisor can see how your chest is currently supported.

Back extenders
It is possible to buy a back extender to add to a bra you already have, if you like it. During pregnancy, your width changes as well as your cup size - sometimes much more signifcantly than your cup size - so this can be useful for women who don't want to change their entire lingerie collection.

Room to grow
Cup sizes between American and European models can vary slightly, so getting a fitting can be useful. With the back hooks, buy a bra that fits your comfortably on the tightest securing so you have extra hook widths to grow into as your bump gets bigger and your ribcage expands in preparation for birth.
If you are buying a maternity bra quite close to the end of your pregnancy, buy one where your body fits well with the widest hook setting, as your ribcage will go back to a smaller size after birth.

Night support
Even if you find that your underwired bra is working fine during the day without putting too much pressure on the underneath of your growing bosoms, you should not wear it as support at night.
Instead, companies like Emma Jane (supplied by outlets including Mother Nature Bras, see below) make 'sleep bras' which are softer but still offer good support at night, if you find that an extra pillow or small cushion is not enough to keep you comfortable.

Useful websites and companies
Mother Nature Bras have useful advice about maternity and nursing (feeding) bras, as do Bosom Buddies.
There are many good suppliers of bras, including Figleaves (who also offer some great advice) and Isabella Oliver.