Asymmetry is the norm, not the exception

Breasts are not matched pairs. They differ in the amount of glandular tissue, in the number and size of ducts, in nipple and areola shape, and in how easily milk flows. Any of those will make one side produce more, and most people notice it — often when they start expressing and see two very different bottles. The Australian Breastfeeding Association's discussion of breast size makes the underlying point: what determines milk production is glandular tissue and how often milk is removed, not the outward size of the breast.

A commonly repeated observation is that the right side produces more for most people. Whatever the reason, a noticeable difference between sides is not a fault to be corrected.

Why a baby prefers one side

The HSE lists the usual reasons, and they are worth knowing because several are fixable:

A physical cause — for example a sore head or arm after a difficult birth, or torticollis, where the neck muscles are out of alignment and turning one way is uncomfortable.
Positional preference from the way your baby lay in the womb.
Differences in nipple or areola shape that make one side harder to latch onto.
A difference in milk flow — a fast side that overwhelms them, or a slow side that takes more work.
Your own comfort. If one side has been sore, you have probably favoured the other without noticing, which then increases the difference.

Once a preference starts, it reinforces itself: less feeding on one side means less milk, which makes that side less rewarding, which means less feeding.

What actually helps

The HSE's practical suggestions are the ones to try first:

Keep the position, change the side. If your baby feeds well in cross-cradle on the right, slide them across to the left in a rugby-ball hold so their body is in the same relationship to you. This is the single most effective trick.
Start feeds on the less preferred side, when your baby is hungriest and most willing to work.
Or offer it when they are least hungry — some babies accept a slower side better when they are not desperate. Try both and see which your baby prefers.
Offer it in light sleep, when they are less aware and less fussy.
Offer it while walking or rocking, using the distraction.
Skin-to-skin before feeds, and laid-back positions, which work well for babies with a sore head or arm.
Express the less preferred side regularly while you work on it, to protect the supply on that side.

When to get it checked

The HSE flags one situation as needing advice: a baby who has been feeding well from both breasts and then suddenly latches to only one side. There may be a medical reason — an ear infection, nasal congestion or soreness on one side. It also notes that if you have had mastitis, the taste of your milk on that side may have changed or the flow may be slower, and that this usually passes.

Can you feed a baby from one side only?

Yes. The HSE says so directly: many mothers have exclusively breastfed babies on one side, and in most cases it is possible to make enough milk, though it depends on the milk-making and storage capacity of that breast. A baby feeding on one side may need to feed more often than a baby taking both. This is worth knowing if you have had surgery on one side, or if one side has never produced much.

The HSE also notes the cosmetic consequence and its resolution: your breasts may end up noticeably different in size while you are feeding on one side, and when you stop breastfeeding they will return to about the same size as each other.

The risks of leaving it entirely alone

The HSE notes that long-term one-side preference can lead to low or no production on the less preferred side, and to engorgement and mastitis on the side that is being over-used. So it is worth some effort — not because lopsidedness is a problem in itself, but because an unused breast can become sore and an over-used one can become blocked.

What not to worry about

Visible lopsidedness. It is temporary and it is not a health problem.
Getting far less from one side when pumping. Expected, and not a measure of your overall supply.
Your baby taking one side and refusing the second at some feeds. Common, especially in the early months, and fine as long as weight gain and nappies are good.

When to ask for help

Speak to your midwife, health visitor, public health nurse or an IBCLC lactation consultant if your baby suddenly refuses a side they used to take, if the difference is affecting weight gain, if the unused side becomes red, hot or painful, or if you feel a lump that does not settle. A lump that remains after a feed, or after your milk has gone, always needs checking.

After surgery or an injury on one side

If you have had a biopsy, lumpectomy, an abscess drained or radiotherapy on one side, expect that side to produce less, sometimes much less. Feeding on the other side alone is a workable plan, and the treated side may still make some milk. Tell your infant feeding team, because they can help you judge whether the working side is keeping up rather than leaving you to guess.

Evening it out, if you want to

If the difference bothers you cosmetically rather than practically, adding a short pump session on the smaller side once a day, or always starting feeds on that side, will narrow the gap over a couple of weeks. It is entirely optional. Plenty of people simply carry on lopsided for a year and find it resolves after weaning, which is what the HSE describes.

One Breast Produces More Milk Than the Other: Why