Soon as I found out I was pregnant..
I was about 3 almost 4 months my breasts became larger filling with water balloons
I'm 15 weeks had my bra fitting last week and have gone from a D to G. I was speaking to a lady who is due in Feb and she has only just noticed change so don't stress to much! You'll soon know when the boob pains start!
Im 25 weeks and barley noticing it. Everyone's body is different
Immediately for me, like 3 weeks, but of course the big changes didn't happen until after my son was born. From a small 34B to 34C during pregnancy then 3 days postpartum went to 36E overnight. After two years of breastfeeding they stuck at 36C so who knows what'll happen to them this time around. They're definitely bigger and sore right now (10w) but I'm surviving on sports bras so don't know how much they've grown just yet.