Lactation Basics (how your breasts make and release milk)
Learn the Basics
Before you begin to breastfeed, take some time to learn the basics of how it works. Lactation is unlike any other bodily process, and knowing more about it will allow you to better understand how to do it properly and how to avoid (or at least be ready for) some common problems like a shrinking supply or clogged milk duct. Wikipedia has a decent lactation entry. Other informative descriptions of lactation come from Kellymom and the La Leche League. One of the better discussions I have found online is by pediatrician Carol Wagner and available here on the emedicine blog from WebMD. The basic biology and key terms are described here.
Your body prepares to make milk late in pregnancy. Your breast cells become more specialized and prepare to make large amounts of the characteristic components of breastmilk, such as lactose and immunoglobulins. This phase is referred to as Lactogenesis I. You begin producing milk after your baby is delivered. The sharp decrease in pregnancy hormones (namely progesterone) after birth initiates your milk production. Once this process begins, it requires 2-3 days for your breasts to feel full. This initial feeling of fullness and build up of milk is what women refer to as their milk "coming in." It is important to note that giving birth stimulates this phase, Lactogenesis II, not nursing. Your breast will make milk at this point whether you nurse or not. Please remember that just because your milk comes in quickly and/or copiously does NOT mean that your baby is latching and nursing well (and vice versa). Keep a close eye on your production as this "automatic milk" produced after birth goes away and your supply begins to depend on nursing properly. Once your body has become accustomed to the cycle of producing and releasing milk, you have reached the phase known as Galactopoiesis - this is kind of like finishing a marathon - congratulations!
Releasing Milk (let down)
You produce the components of breastmilk in specialized mammary cells. It is released from these cells into small ducts that gather into larger ducts. These ducts carry milk to about 20 openings at the tip of your nipple. As your breast releases milk, the milk sprays out from these openings. It is important to remember that milk flows from your breast into your baby's mouth not so much because it is pulled out by sucking, but more because it is pushed out by contracting breast cells. When these cells contract, milk flows, or even sprays forcefully, through your nipple. This is referred to as "let down." Let down does depend on nursing. That spraying milk is a response to signals from the brain (primarily the hormones oxytocin and prolactin) produced in response to your baby's sucking (or a breast pump, if necessary). To keep making milk from this point on, you must either nurse or pump at regular intervals. Feedback inhibition, a biological system for halting production of things that buildup to a certain level, is another important factor. This keeps full breasts from making more milk, so it is important both to stimulate milk release and to empty the breast.
Observing the Process
A good breast pump, which is helpful in numerous ways throughout breastfeeding, will allow you to see clearly how this works. Initially, milk dribbles gradually - drawn out by the sucking pull of the pump then, after a minute or two, you will begin to feel your breast contract as let down begins. Milk then sprays out in many directions from your nipple. The let down response is more extreme in the new mother. This can cause your bra/shirt to be soaked during feedings by the unnursed boob, unless you wear a breastpad - especially early on. If too much milk (enough to soak a breastpad) is lost, it may be useful to use a catch to collect the the milk. This will help you ensure the milk goes to the baby and isn't soaked up in the breastpads!
Take home summary
Having a baby will initiate milk production. This initial production, however good, will dwindle and stop without stimulation of the newly active breast by a baby or a breastpump. Nursing your baby (or pumping with a high quality motorized pump) does not simply draw milk out, it stimulates your brain to produce key hormones that cause the breast cells to contract and spray milk. Effective nursing will stimulate milk release and empty the breast so that more milk will be produced. If your baby has trouble nursing in the first days or weeks, a pump can help (see Pumping Sucks).