Your Breasts Increase By 2-4 Cup Sizes: Breast size increases by 1 to 2 cup sizes during pregnancy. Some women’s cup size will increase even more. Soon after delivery, milk begins to fill the breasts and can increase breast size by another 1 to 2 cup sizes. Your lung capacity also increases during pregnancy to help you get more oxygen for you and your baby. This can increase torso size, requiring a larger bra band size. It is estimated that 80% of women wear the wrong bra size during pregnancy. So ladies, go get a bra fitting and make yourself as comfortable as possible during this period!