That's milk production starting up. Lasts till the baby comes and then your norks turn into torpedos! Sorry I can't give you better news. However if you get itchy all over tell the midwife in case its ostetric choliasis (or however you spell it).
i am 13wks preg and really itchy boobs too, very annoying, i had it during my last pregnancy too tho, so i guess it can a be a normal side effect, not one i have found mentioned in the preg magazines etc
it may be worth checking that you dont have thrush on your nipples tho, or have you recently bought anew bra? that could be causing a reaction, your skin is super sensetive when preg and all sorts of this can trigger itching etc.
i have never heard it said that they were itchy cos of milk production, i am still bfeeding my littlest one, so they never stopped producing milk, i wondered about thrush, but its def not that, had it checked
hope they feel better soon,you could always try cold cabbage leaves in your bra to soothe them.
I had this quite early on in my first pregnancy. My mw couldn't tell what caused it, but she said it might be sensitive skin (which I have) and advised to start wearing maternity bras. I don't personally think that it made any difference at the time, but the itching did stop a few weeks later.