As your baby grows during pregnancy, these muscles begin to stretch (which is normal). (proactiveph.com)
You may have watched your breasts grow during pregnancy as your milk supply built up, preparing to feed your baby. (romper.com)
Your breasts grow during pregnancy because estrogen and progesterone are adding fatty tissue to your boobs and increasing the blood flow to get ready to produce milk, notes What to Expect. (romper.com)