Breastfeeding the baby offers protection, immunity against various diseases and usually babies prefer only one side of the breast.

Midwife Cath Curtin, from Melbourne, Australia, explained this phenomenon on Mama Mia's Year One podcast. "A lot of women lactate better on one side," she said. "For some women, one [breast] is really big and one is really small."
"I've looked after women whose breast tissue has only developed on one side and they've breastfed babies really well on the one side," she added.
Behavioural psychologist Karen Pryor and Nursing Your Baby author Gale Pryor confirmed on Baby Center that it's perfectly normal for babies to prefer one boob over the other.
"This tendency is harmless," they wrote on the site. "Your breasts will adjust to produce different levels of milk according to the frequency with which each is nursed."
They also noted that there's a benefit to letting your little one finish one breast before switching: "He'll get the rich, high-calorie hindmilk that comes at the end of each feeding."
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Source-Medindia