The breasts, which develop at puberty , are glands that covers the pectoral muscle and are surrounded by adipose tissue . Each mammary gland is formed of 20 lobes arranged vin bunches . The breasts grow large during pregnancy and produce milk after childbirth when stimulation by a hormone ,prolactin . The mother is milk is routed by the mammary ducts to reservoir , the lactiferous sinuses , where it is stored until it is ejected from the nipples through tiny orifices