Breast buds begin developing in female embryos just four weeks after conception. By the time of birth, basic breast development is complete. The nipples, areolas, and even some milk ducts are in place along with a small pad of fat. Everything is functional on a very small scale.
From just after birth until puberty, breast development is almost on hold. Some milk ducts and glandular tissue grow, but the process doesn't really take off until 10 to 14 years of age.
At the onset of puberty, a hormone called estrogen is secreted by the ovaries, bringing about a rush of breast development. The mammary fat pad increases in size, and the milk ducts grow longer and branch out. When the menstrual cycle begins, the hormone progesterone causes the development of breast alveoli, the milk-producing cells.
Some babies leak breast milk at birth. In the past, people called it “witches’ milk.” Far from being some kind of unnatural event, such leakage is normal and not uncommon. The pregnancy hormones in your body that prepare you for lactation can also cause your newborn's breasts to produce milk. The hormones typically leave the baby's body in a few days and the symptoms pass.
By the age of 20, the process is nearly complete. However, breasts actually continue to mature until either pregnancy or between the ages 30 to 35. Some women may wonder why, with all this growth, their breasts aren't larger. The important thing to remember is that breast development is your body's way of preparing you to nourish a baby.
The size of your breasts does not affect your ability to produce milk. Breast size is actually determined by heredity and body fat, and has very little to do with your ability to nurse your baby. Childbirth naturally completes the cycle of breast development.