Though breast growth is not visible until puberty, breast development begins very early in the embryo and can be discerned within just a few weeks of conception. Interestingly, the earliest stages are identical in male and female fetuses, so many men could develop fully functioning breasts given the right hormonal conditions [www.QinSen.com]
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After birth the breast has only two phases of development; the first at puberty with the outpouring of the hormones oestrogen and progesterone; the second during pregnancy and lactation, when the milk-producing lobules become larger [www.QinSen.com]
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If puberty is stunted or if a woman remains childless, her breasts will not fully develop. The first stage of breast development begins in the embryo at about six weeks, with a thickening in the skin called the mammary ridge or milk line [www.QinSen.com]
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By the time the fetus is six months old, this extends from the armpit to the groin, but it soon dies back, leaving two breast buds on the upper half oft he chest. Occasionally, rudimentary mammary glands develop along the milk line forming additional nipples or breasts that sometimes persist into adult life. More rarely, the two breast buds fade away with the rest of the milk line, so that the nipples are absent from birth [www.QinSen.com]
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Because the initial development of the milk line is the same in male and female fetuses, this development can appear in the male and the female.

When a female fetus is about six months old, 15 - 20 solid columns of cells grow inward from each breast bud. Each column becomes a separate "sweat" or exocrine gland. With it’s own separate duct leading to the nipple

By the eighth month of fetal development, these columns of cells have become hollow so that, by birth, a nipple and a rudimentary milk-duct system have formed. No further development takes place until puberty

The first external signs of breast development appear at the age of 10 or 11 - though it can be as late as 14 years. The ovaries start to secrete estrogen leading to an accumulation of fat in the connective tissue that causes the breast to enlarge. The duct system also begins to develop, but only to the point of forming cellular knobs at the end of the ducts

As far as we know the mechanism that secretes milk doesn’t develop until pregnancy. Although the breast may appear fully grown within a few years of puberty, strictly speaking, their development is not complete until they have fulfilled their biological function - that is, until the woman carries a pregnancy to term and breast-feeds her baby, when they will undergo further changes

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