Breasts of different sizes

Is it normal to have one breast bigger than the other?

Many women may have one breast that is slightly larger than the other. This is normal and mothers should treat the breasts equally by changing the starting side at each feed. Having breasts that are very different in size may be caused by a breast abnormality and they may produce very different amounts of milk.

A) Normal size differences between breasts

Women may have one breast that is slightly bigger than the other and the bigger one can be either the right or the left. This is normal.
 
Studies have found that in about 50% of women the right breast produced more milk, in 25% of women the left breast produced more, and the remainer had similar amounts of milk in both breasts (Engstrom 2007). This may be related to handedness and breast preference as more individuals are right-handed.
 
Mothers should change the starting breast at each feed to remove milk from both breasts equally and prevent one breast from producing more milk and possibly becoming larger, and the other less milk and becoming smaller.

B) Large differences in size between breasts

If the breasts look very different in size, the smaller breast may not make as much milk. There may have been a problem in the smaller breast occurring before the baby’s birth, preventing it from ever making enough milk. It is also possible that a problem developed after the baby’s birth.

Having one breast with a low milk supply can lead to the baby not taking in enough milk. In addition, babies frequently prefer the more active breast and may become reluctant to breastfeed on the weaker side or may even reject it completely.

References

Engstrom JL, Meier PP, Jegier B, et al. Comparison of milk output from the right and left breasts during simultaneous pumping in mothers of very low birthweight infants. Breastfeeding Medicine 2007; 2(2):83-91