It’s science: You’ve got a slacker boob

Ask a breastfeeding mama if she has one breast that produces more breast milk than the other, and she’ll likely say yes—it’s the one on the right. That left breast is trying so hard to keep up—and its efforts really are herculean. But the truth of the matter is that when it comes to biology, the left breast often just can’t keep up with the right.

Many women have one breast that produces less milk during feedings—sometimes referred to as “uneven milk supply” or (unofficially) “slacker boob.” And contrary to what you might think, this uneven milk supply is unrelated to whether you are right- or left-handed, or whether you carry your baby on your left side.

In a study published in the Breastfeeding Medicine, the amount of breast milk pumped by more than 200 mamas was measured. It found that differences in the amount of output are common from the right and left breasts, and that breast milk output from the right breast was greater in 65.7% of the sessions, regardless of left- or right-handedness.

This happens because both breasts are not exactly the same.

According to Dr. Bradley Bengtson, a plastic surgeon in Grand Rapids, MI, “Breasts are sisters, not twins.” Different characteristics of each breast can make a real difference in how much milk they produce. For instance, it’s common for mamas to have different amounts of milk-making tissue and different sized milk ducts in each breast, so one breast naturally produces more than the other. Also, the structure of the nipples can vary, making it easier for your baby to latch on to the nipple of one breast than the other.

Another reason one breast may produce more milk is because your baby just prefers one breast over the other.

Most babies show a preference for one side or the other and will nurse more efficiently from one side. The amount of breast milk produced depends on the amount of breast milk removed, so the breast that is not “used” might not produce as much milk. Based on this, some mamas feed more on one side, causing one breast to get fuller and produce more milk than the other.

Even though both breasts don’t produce the same amount of milk, don’t worry: Your body knows how much breast milk your baby needs.

As long as you nurse on demand, your baby will get enough breast milk to fulfill their nutritional needs. But if you are concerned, here’s how to encourage your slacker boob to make more milk:

• Frequency matters, especially early on in breastfeeding. If you notice a difference in production, try to nurse more often to balance them out.
• Offer the least desired breast first with every feeding—your baby will usually nurse more from the first breast offered.
• Pump between feedings, particularly on the side that is making less. More stimulation will increase demand and hopefully increase breast milk production.
• Drain your breasts as thoroughly as possible each time you nurse or pump. Breast milk contains a small whey protein called Feedback Inhibitor of Lactation (FIL) which seems to slow milk synthesis when the breast is full. So breast milk production slows when it accumulates and more FIL is present, and it speeds up when the breast is emptier and when less FIL is present.

Breast milk may taste different (usually saltier) in one breast if it has a lower supply than the other. But that doesn’t mean the milk from your slacker boob is not as nutritious. In a small study published in the Journal of Perinatology, researchers compared the macronutrients of breast milk from both breasts of mamas in the study and concluded that the nutritional content of the breast milk was unaffected by their dominant hand, breast or breast size. So even if one of your breasts produces less breast milk, the other breast will produce enough breast milk to make up for the slacker boob.

And the truth, mama? Your slacker boob may produce less milk, but there is nothing about you that is slacking off. Your boobs are working as hard as they can (just like you) and you should be immensely proud—yes, even of slacker leftie.

Editor’s note: If you are concerned about pain, size, production or anything else, visit your local La Leche League. They can assess the situation and advise accordingly, and their services are free. It also may be helpful for you to see a lactation consultant. They can be extremely helpful in helping with many breastfeeding issues.