take in extra oxygen for yourself and the baby

Unexpected Effects of Pregnancy

Bra Size

An increase in breast size is one of the first signs of pregnancy. Breasts usually become swollen and enlarged in the first trimester because of increased levels of the hormones estrogen and progesterone. That growth in the first trimester isn't necessarily the end, either — your breasts can continue to grow throughout your pregnancy!

In addition to the size of your breasts, your bra size may be affected by your rib cage. When you're pregnant, your lung capacity increases so you can take in extra oxygen for yourself and the baby, which may result in a bigger chest size. You may need to replace your bras several times over the course of your pregnancy.