I just wanted to share my experience today 
I'm 17 weeks and have been persevering with my current Bravissimo 28E bras by popping an extender in the back. I decided enough was enough though as every night I'm left with sore red marks so I went along to John Lewis at first. Not only did the assistant insist that my breasts will not grow any bigger during the pregnancy because they stop growing at 12 weeks
(my breasts have definitely increased in the last week) but that I also HAVE to wear non-wired bras. She fitted me with some very unflattering 34D t shirt bras which looked horrendous under my clothes 
I then went to Bravissimo, was told I can wear under wired as long as I haven't been told otherwise by a medical professional and so therefore I could stick to my favourite Satine style. Hoorah! I have come away with two very comfortable and pretty 30FF bras.
I'm a bit gutted because the bravissimo bras are twice the price of john Lewis but I couldn't bring myself to live in those monstrosities for the next few months. Who decided that pregnant women have to keep away from attractive underwear?