It is not always possible to find a reason for preference of one breast. Sometimes the baby prefers it this way, because he finds it more convenient and that's it. However, sometimes this choice can be explained as follows:
milk flows easier from the chosen breast;
at the beginning of feeding you latched the baby on more to one breast, for example, because it was more comfortable to you for some reasons (the postpartum wound hurt less, or the cannula or drip got less in your way) and now this more frequently given breast "functions" better (milk flows easier, is more abundant), or the baby simply got used to that position;
disliked breast has a nipple of a different shape and is harder to grasp;
preferred breast produces more milk;
baby selects the breast at which he lies more comfortably.
Although the reasons for preferring one breast can be insignificant and temporary, it is always worth telling your doctor to exclude any possible, more serious causes of the baby's aversion to suckling the other breast. The reasons below require more attention:
The baby cannot suck the other breast because his breast is constantly turned to one side. This may only be a physiological postural asymmetry, but the reason can be more serious and requiring rehabilitation.
The baby cannot suck from the other breast because when he lies on that side, he feels pain, maybe due to ear infection (however, this may simply be caused by arm or thigh sensitivity following vaccination).
The rejected breast produces much less milk and this condition is not remedied despite regular extraction with the pump. Tell the doctor about that. It is necessary to exclude any of your illness that could be responsible for this condition.
Patiently and tenderly encourage the baby to suck the rejected breast:
Experiment with breastfeeding in different positions. When the baby refuses suckling held in the cradle hold, give him the rejected breast holding him under your arm or when lying down. You can place the baby beside you on a higher pillow so he can reach the rejected breast while lying on his favorite side.
Propose the less liked breast to the baby when he is hungry (e.g., after longer sleep or a walk), maybe hunger will make him empty the given breast.
When the baby rejects the breast because of a flat nipple, check what you can do to modify its shape and possibility to grasp it, using tips provided in the chapter Flat nipples).
Extract milk from a rejected breast to maintain lactation in it.
Yes. Breastfeeding with only one breast will harm neither you nor your baby. The breast preferred by your baby will produce milk in quantities sufficient to cover his needs. However, as long as there is a chance that your baby can be convinced to suck the disliked breast, it is worth to maintain its lactation. But if nothing works and the baby firmly refuses to suck the rejected breast, allow lactation to cease. For some time, extract from it enough milk not to feel tension, and after extraction apply crushed, cooled white cabbage leaves. After a few weeks the unused breast will stop to produce milk, but this will not affect the successful breastfeeding with the other breast at all. The unused breast will now be much smaller than the breast still sucked by the baby; however, this disproportion will disappear when you wean the baby completely.