The lactation system inside your breasts resemble a tree. The milk glands (the leaves) are grapelike clusters of cells high up in the breast that make milk. Milk travels from these glands down through the milk ducts (the branches). These ducts then widen beneath the areola (the dark area surrounding the nipple), forming milk sinuses (the tree trunk), which then empties into the approximately twenty openings in your nipple (like the channels going down to the roots of the tree). These milk sinuses are located beneath your areola.
To empty these milk sinuses effectively,your baby's gums must be positioned over them so that baby's jaws compress the sinuses where the milk is pooled. If baby sucks only on your nipple, only a little milk will be drawn out, and your nipple will be irritated unnecessarily. Remember the golden rule of effective latch-on: Babies suck on areolas, not nipples. Baby must have enough of your areolas in her mouth to get the milk out.
Your baby's sucking stimulates nerves in your nipple that send messages to the pituitary gland in you brain to secrete the hormone prolactin. Prolactin surges encourage continued milk production, which goes on around the clock. As your baby continues sucking, the sensors in your nipple signal the pituitary gland to secrete another hormone, oxytocin. This hormone causes the elastic tissue around each of the many milk glands to contract, squeezing a large supply of milk through the milk ducts into the sinuses and out the nipple. This is called the milk ejection reflux, or MER. The milk may come out so fast that it leaks out the side of your baby's mouth. If you were pumping or expressing by hand, you would see the milk spray out in every direction.
The first milk your baby receives at each feeding is the foremilk, which is thin like skim milk because of low fat content. As baby continues to suck, more oxytocin brings on phase two, squeezing out the later milk (called hindmilk), which is much higher in fat and slightly higher in protein and, therefore, helps baby gain weight and helps baby's tummy feel full. Consider this creamier hindmilk "grow milk."
The more milk that is removed from your breasts, the more milk your body makes to replace it. Frequent removal of milk from your breasts by your baby or by a pump will sti9mulate your body to produce more milk. When your baby breastfeeds less, the body responds by cutting back on milk production. This supply and demand system is how mothers produce enough milk for twins or even triplets.
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