How it all begins
Believe it or not, breast development begins within the wound as early as the 4th week of life as a fetus. Breasts are ectodermally derived and first begin as thickenings of the epidermis that extend from the armpit to the groin forming the 'mammary lines' or 'milk lines' . By the 9th week, the milk lines are reduced to the chest area leaving only 2 bud like structures. Primitive vessels develop and near the 13th week the nipple as well as areola appear.
The mammary gland is present as a disk of about 1 cm in thickness. From here on in the breasts remain dormant while the mammary glands change with lobes and galactophores (ducts in the breast) beginning to form .
The stages just before development and during are summed up into 5 stages as originally proposed by Tanner :
Stage 1: Preadolescent: Tip of the nipple is raised
Stage 2: Buds appear, now the breast is raised as well. The areola enlarges
Stage 3: Breasts are a little larger and some glandular tissue is present
Stage 4: Areola and nipple are now raised which form a mound
Stage 5: Mature Adult: Breast round and only the nipple is raised