Why is one breast bigger than the other?

Ask Anne…

Question:  One of my breasts has always been a bit larger than the other, but since I started breastfeeding, the difference is really pronounced. Other than the way it looks, my main concern is that my left side is overflowing, while  my right makes barely anything.Why is one breast bigger than the other, and is there anything I can do about it?

Answer: It’s not uncommon for one breast to be larger than the other, and for one to produce more as well. The two breasts are independent, and aren’t like mirror images of each other. That’s why most moms who double pump will get different amounts from each breast, and also why so many babies develop a preference for one side over another.

In some women, it’s a subtle difference, in others it can be dramatic – five or six ounces from one breast and one from the other .

Because anatomical differences account for the discrepancy in size and production, then extra stimulation (via nursing or pumping) on the smaller side usually doesn’t make much difference. I would watch the larger breast carefully for signs of plugged duct and mastitis (breast infection) while you are in the “settling in” process of building and establishing your supply. This process usually takes around six weeks, but for some moms it takes less time, and for some it takes longer. After 6 weeks, most mothers will notice that their breasts aren’t as full, and they have less leaking than they do in the beginning.

Other than looking lop-sided, it shouldn’t cause a problem, unless you develop a breast infection (see  Breast Infections and Plugged Ducts). Women with only one functioning breast can still produce plenty of milk for their babies. Babies will take what they need, and your breasts will respond to supply and demand. There is a small percentage of women who continue to have problems with engorgement on one side even after 6 weeks, and some of them decide to let the problem breast dry up completely and nurse exclusively on one side. You may want to try cabbage leaf compresses (see “Engorgement“) on the larger breast and see if that is helpful.

Once your baby weans, things should go pretty much back to where they were before, except that the hormonal and anatomical changes that occur during pregnancy stretch the tissue and they will never be as firm again. This happens even if you formula feed.  You may always notice a size difference, but it shouldn’t be as pronounced as it is while you’re lactating.

Anne Smith, IBCLC
Breastfeeding Basics