Newborn babies of both genders can have swollen breasts that even produce small amounts of milk. This infant lactation is caused by the maternal hormones acquired through the placenta, which stimulate the baby’s mammary glands. The lactation will markedly decrease or disappear a few weeks after birth. Later on, girls may have mild swelling of the breast buds years before puberty. As long as these breast buds are small and unaccompanied by other signs of sexual development, such as pubic hair, they are not worrisome.