What causes enlarged breasts in a newborn?

My sister's newborn daughter has enlarged breasts. What would cause this and is it normal?

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Robert Steele

Robert W. Steele, MD, is a board certified pediatrician at St. John's Regional Health Center in Springfield, MO. He graduated from medical... Read more

Breast development is influenced by the sex hormone estrogen. The levels of estrogen vary significantly at different times during life. And its effects cause changes throughout the body. These changes include maturation of mucous membranes of the vagina, breast enlargement, and bone maturation. Therefore, when concerned about early breast development, the question is not necessarily "Is my child going through puberty early?" but rather, "Where is the estrogen coming from, and is it normal?"

The fetus grows in an environment rich with hormones. In fact, newborn girls may show signs of this exposure by having large labia around the vagina, breast buds, which may produce colostrum, (the same initial fluid breastfeeding moms secrete) and even some vaginal bleeding. This is due to the levels of estrogen being about as high as a normal girl going through puberty. However, these initial signs generally subside within the first few weeks after birth because these hormones passed from mother to daughter go away.

However, over the next few months, the infant's own hormone system begins to mature. This maturation involves the signals the brain gives the ovaries to make sex hormones. It is at this time that estrogen levels go up again peaking at about four months of age. This increase in levels may be enough to sustain breast development through early infancy. Over the next one and a half years, this system undergoes further maturation allowing for the inhibition of these signals, and the breasts stop their development or regress.

The process described above is the most common cause of breast development in an infant and should be considered normal. These changes are due to estrogen and should be limited to this.

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