One may be bigger

Don’t freak out if she appears a little lopsided: About one quarter of all women have one breast that is larger than the other, says Keri Peterson, M.D., an internal medicine physician and Women’s Health medical contributor. “Women’s breasts are independent beings that respond to estrogen differently during puberty,” Peterson says. For about 65 percent of women with breast asymmetry, the left dominates. A Turkish study suggests that the left side of her body has a higher immune hypersensitivity, a factor that impacts hormones such as estrogen that control breast shape and size. This unevenness is only cause for concern if one breast abruptly becomes larger than the other, Peterson says. If that’s the case, she’ll likely notice the change like you will—feeling lumps or increased tenderness, which could be signs of infection, cysts, or even breast cancer.