There are many hormones that play a role in the start-up of milk production, a process called lactogenesis. Like a good cake recipe, some ingredients (hormones) play minor roles while some are crucial for a good result. Prolactin, the major milk-stimulating hormone, is normally present in small amounts in our bodies. During pregnancy, prolactin gradually reaches very high levels, multiple times its normal level, peaking at the end of pregnancy. The only reason that volumes of milk are not made before birth is that high levels of progesterone interfere with prolactin receptors (“locks”) by not allowing them to multiply.  This prevents the prolactin “keys” from having much effect and allows only small amounts of the first milk, colostrum, to be made. This is known as Lactogenesis I, the time during the second half of pregnancy when only colostrum is produced. Since the placenta makes the majority of the progesterone during pregnancy, once it is delivered after birth, the progesterone level drops quickly, allowing prolactin to start doing its work. Within 30 to 40 hours(3) after birth, the change to full milk production begins.  Around day two to four on average, mothers notice that they are producing more milk that is lighter in color and say that their milk has “come in.”  This stage is known as Lactogenesis II. There are three major hormones considered necessary for Lactogensis II to begin, sometimes referred to as the Lactogenic Complex: prolactin, insulin, and cortisol.(4)

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Nature is amazing in how it is constantly tailoring your milk to be just what your baby needs. Colostrum is made during the second half of pregnancy and is the first milk that baby receives after birth. Unlike the later mature milk, it is yellowish in color because it contains a lot of the nutrient beta carotene, which is an anti-oxidant and a building block of Vitamin A.  It is thicker in consistency and small in quantity; babies take between one half to four teaspoons (2-20 mls) per feeding for an average total of 3.38 ounces (100 mls) per day. Colostrum is higher in protein and lower in fat than mature milk, and is densely packed with immunoglobulins, which are antibodies and anti-oxidants that colonize baby’s digestive tract and help inoculate and protect against future bacteria, viruses, fungus, and allergens. Colostrum also has laxative properties that stimulate the bowels to quickly get rid of meconium, baby’s first stool, helping to minimize jaundice in the days after birth.

As full milk production begins when the milk comes in, colostrum gradually lightens in color and becomes more opaque to form transitional milk, which is produced in higher volumes but is still yellowish in color. Transitional milk is halfway between mature milk and colostrum, and gradually increases in fat and volume.  The change from transitional to mature milk lasts from between 7 to 14 days. Mature milk is whiter (though it can vary in color according to mother’s diet) and has the most cream while being 88% water. It contains many immune properties, but they are not as concentrated as in colostrum.  While the recipe for mature milk remains fairly constant, human milk continues to change in smaller ways as baby gets older to meet his ever-changing needs.(5)

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You may have heard that mothers produce two kinds of milk, foremilk, the thinner milk the baby gets first, which has a lower fat content, and hindmilk, the high-fat, creamier milk that follows. These terms can make it seem as if the breasts produce two kinds of milk, but this is not the case.  The milk-making cells in the breast actually produce only one type of milk, but the percentage of fat in the milk that is removed varies according to how long the milk has been collecting in the ducts, and how much of the breast is drained at the moment. As milk is made, the fat globules stick to the sides of the alveoli where the milk is stored, while some of the watery portion of the milk moves down the ducts toward the nipple, mixing with any milk left there from the last feeding. The longer the amount of time between feedings, the more diluted that leftover milk becomes. This “watery” milk has a higher lactose content and less fat than the milk stored in the alveoli higher up in the breast. As baby begins nursing, the first thing he receives is this lower-fat foremilk, which quenches his thirst. Baby’s nursing triggers the mother’s milk-ejection reflex which squeezes milk—and the sticking fat—from the alveoli into the ducts. The longer he nurses, the higher the fat content of the milk and the more cream he receives, ending up with a nicely balanced meal containing all the fat calories for growth and lactose for energy and brain development that he needs.(6), (7)