One thing’s for sure – no two breasts are alike.  Even on the same woman, breasts can vary slightly in size and shape.  In fact, one hundred percent of women have some degree of breast asymmetry or uneven breasts. Breast asymmetry is defined as a difference of form, position, or volume of the breast.  But no worries – asymmetrical breasts are a common condition and usually not a cause for concern.

To understand breast asymmetry, it is important to understand the anatomy of the breast itself.  The breasts are actually glands contained within the front of the chest wall.  The average breasts weighs between 7-10 ounces and are primarily composed of 12 to 20 lobules that spread out from the nipple like the spokes in a bicycle tire.  These triangular lobes each have one central duct that opens at the nipple through which milk exits. 

Natural changes that happen to breasts

Changes in breast shape begin with the physical changes of puberty for girls.  Breast development, is the first earliest sign of the arrival of puberty which usually starts around ages 10 or 11. But some girls may notice the appearance of breast development as early as age 7.  When breast development gets underway, it is not uncommon for one breast to start growing before the other due to hormonal changes occurring.

It is also perfectly natural for adult women to see noticeable differences of their breasts.  For example, breasts are known to fluctuate in size during the time of a woman’s menstrual cycle.  During ovulation, many women may notice that their breasts feel fuller and are more sensitive. The reason for this change is because of water retention and additional blood flow.   Then once menstruation begins, the breasts shrink back to their normal shape and size.

Pregnancy is another natural state for women when the breasts may appear uneven.  This is very likely due to hormonal changes and the body preparing for the breasts for breastfeeding that will make the breasts larger and sometimes lopsided.  Once the baby is born and is breastfeeding, a more definitive but again normal change is how the breasts will often be uneven in size, particularly if the baby is favoring one breast over the other.

Unusual causes of breast asymmetry

There are a couple of medical conditions that can affect breast symmetry: 

One is called atypical ductal hyperplasia, or excessive growth of tissue in the breast’s milk ducts.  It is not a form of breast cancer but is considered a precancerous condition that is a marker for women who may have a risk factor for developing breast cancer in the future.  Women with this condition may have more cells that line the duct than normal with some of these cells being irregular in shape and size.  This unevenness can lead to benign lumps within the breasts which can affect the symmetry of the breasts. 

Another rare condition is hypoplastic breasts.   This is also known as underdeveloped breasts as in breasts that have insufficient glandular tissue or not as many milk producing glands and does not have anything to do with the size of the breasts.  Women with both small and large breasts can have hypoplastic breasts. 

Signs indicating if a woman has hypoplastic breasts can be a wide space between each breast (no cleavage), asymmetrically sized breasts, stretch marks on the breasts, having a large areolae, and if no changes occur in the breast size when a woman is pregnant or immediately after birth.    This means the breasts do not have as many milk producing glands in them as other women.