The causes of breast asymmetry

Updated February 21, 2017

Asymmetrical breasts cause embarrassment, frustration and concern in many women, even though it typically should not. Many women identify their femininity with their breasts and the slightest imperfection threatens the entire psyche. Breast asymmetry affects nearly 50 per cent of women, making this condition just as normal as symmetrical breasts.

Puberty and Genetics

Puberty triggers the production of oestrogen, which in turn stimulates the growth of breast tissue. Doctors believe genetic differences and different growth rates contribute to one breast growing larger than the other. Typically, breast growth begins approximately two years after the first menstrual period and continues for another two to four years. This is typically when most girls begin to notice a difference. After the age of 21, breasts are usually done growing and if a difference is still present it likely will remain.


Hormone-based contraception can cause breast tissue to grow, and not always in both breasts. Hormone-based contraceptives such as birth control pills rely on unusually high levels of progestin and oestrogen, in some cases 60 per cent more, to regulate periods and prevent pregnancy. The body, and breast tissue specifically, responds to the increased amount of hormones with swollen tissues and glands sometimes causing temporary growth. This should not be permanent and if the asymmetry persists, or you are no longer on hormone-based contraceptives, consult your physician.

Pregnancy and Menopause

Pregnancy and menopause case extreme fluctuations of hormone levels sometimes wreaking havoc on your body and breasts. When pregnant, your body prepares to have a child and supply nutrients through breastfeeding. The increased levels of progesterone cause changes including rapid growth of the milk duct system, swollen glands and sore breast tissue. These are all normal occurrences but they will not necessarily happen at the same rate in both breasts.

During menopause, oestrogen and progesterone levels fluctuate greatly. A significant reduction in oestrogen, as much as 50 per cent in some cases, causes the connective tissue in the breast to dehydrate and lose elasticity. Without the elasticity, the breast will begin to change shape, which may or may not take place in both breasts evenly.

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About the Author

Stephanie Steensma began writing in 1998 as a radio news reporter. Her work has appeared in print publications such as "Engineering Today" and "Dome Magazine" as well as online. Steensma has a Bachelor of Arts in communication and journalism from Western Michigan University.