First trimester
Breasts swell early — often a full cup size by week 12. Switch to wirefree maternity bras.
Second trimester
Get measured around week 16. Expect 1 band size up and 1–2 cups up from pre-pregnancy.
Third trimester (week 36+)
Get fitted for nursing bras. Add 1 more cup size to allow for milk-in around birth.
Postpartum and weaning
Re-measure 6 weeks postpartum, then again 3 months after weaning when milk production stops.