April 17, 2011 -- Breasts are good for much more than creating cleavage -- they're complex physiological structures. At no other time is this more evident than when you're pregnant and nursing. For instance, did you know lactating breasts can swell by more than a pound? Here is a detailed look at the changes your mammary machinery undergoes during each phase of pregnancy.
The areolae take on a darker hue. A deeper nipple color might make it easier later for the newborn to spot his or her food source.
In preparation for the coming feeding frenzy, a flood of hormones stimulates the growth and expansion of the breasts' milk-producing lobules.
Breasts begin producing and storing a type of milk called colostrum, a thick, yellowish substance that will feed the baby for its first few days.
By week 16, the breasts are ready to dispense food. To increase a preemie's chance of survival, milk at this stage is higher in protein, iron, sodium, fat, and anti-infective properties.
At 24 weeks, blood flow to the breasts doubles--and will hold steady until birth--to support the ongoing production of milk.
Tiny milk-making cells inside the alveoli begin to multiply; they'll work on overdrive until they're full of colostrum.
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All that suckling takes a toll! Tiny grooves in each areola, called Montgomery glands, release a lubricant that protects the nipple. That same lube is laced with infection-fighting antimicrobials.
Milk is delivered on a supply-and-demand basis. Suckling stimulates nerves in the nipple that signal the brain to secrete milk-making hormones.
One of these hormones, oxytocin, also helps the uterus shrink back to its prebaby size--which is why new moms often feel abdominal cramps while nursing.
The average bra size in the United States is 36C.
Every breast is divided into about 15 to 20 lobes, or sections, which branch out from the nipple like the spokes of a wheel.
The average areola, the darker circle of skin around your nipple, is three centimeters wide, about the diameter of a banana slice.
Each lobe is filled with 12 to 20 grapelike glands called lobules, and those lobules contain thousands of milk-producing machines called alveoli.
It's like an intricate subway map in there: A network of about 20 tiny tubes, or ducts, connect the lobules to the nipple.
Fat and fibrous tissue accounts for most of the rest of your breasts--there's no muscle. On average, breasts contain 3.5 percent of your body's total fat.
While lactating, breasts can swell by more than a pound.
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