How Human Milk Production Works

Nursing: From the Beginning

During any given menstrual cycle you have an ebb and flow of hormones (obviously).  Starting on the first day of your period your estrogen levels are the lowest of the cycle and are beginning to rise.  They rise steadily until shortly after you release an egg.  They then drop off suddenly and are replaced by high amounts of progesterone.  Progesterone increases your temperature.  This is why the clearest sign that you have ovulated is your temperature spiking. 

For the remainder of that cycle your progesterone stays elevated (as does your temp) and your estrogen stays low.  If the egg is not fertilized, your period starts, your progesterone drops, and your estrogen again starts to rise through another cycle.  If the egg is fertilized, your progesterone stays high, and your estrogen low.

 

Nursing: The Hormones of Milk Production

 

After the birth of the baby, you produce a few new hormones in larger quantities, namely oxytocin and prolactin.  Estrogen has a drying effect on milk (this is why you need to stay away from estrogen-containing birth control while breastfeeding), so after the baby your estrogen stays pretty low (assuming you nurse. If not, you go back to a normal cycle in a few weeks). 

Each time you nurse, both oxtocin and prolactin are produced.  Oxytocin directly inhibits estrogen (keeping it low).  Prolactin encourages milk production.  Between nursings your estrogen begins to rise.  Each time you nurse, and release the oxytocin, estrogen falls again.  This is one of the reasons nursing is so frequent in the beginning (that and tiny tummies), to make sure that estrogen stays low so that you have plenty of opportunity to produce as much milk as necessary.

 

When baby gets older they may start going longer between nursing sessions, or start sleeping through the night, or you introduce solids thereby making baby space out nursings more. When these things happen your estrogen has the opportunity to rise higher before being stamped back down again than it did in the early days of nursing. 

Every woman has a different magical level of estrogen at which her body will ovulate, thus begin to go through the entire menstrual cycle again.  When you've reached enough time between nursings, and let your estrogen rise to the magical level for you, you will begin to ovulate again.  This is why some women will get their period back one month, then baby teethes or gets sick the next month causing them to nurse more often, and they miss a period.  Or why when the periods first come back they can be very irregular. Because baby's nursing is heavy one week and light the next, etc. 

 

This is why they say "your milk supply is largely controlled by hormones the first 3 months, but by supply and demand after that".  Technically it's always controlled by hormones.  It's just that you had a plethora of the right hormones in the first few months, to go with the frequent demand.  But around 3 months your natural post partum hormone high has run out and baby has begun to go longer between feedings.  This is when it's important to offer often (the general rule is at least 7 times in 24 hrs until at least 9 months), to make sure you maintain a good supply to hold you through until at least the one year mark.  By 13 months a baby is unlikely to wean due to low supply.  Until then you must guard your supply very carefully.

Nursing: There are More than Hormones Involved

 

Aside from the hormones, your milk has an enzyme in it that directly inhibits milk production.  This means the longer you leave milk in your breasts, the more this enzyme is building up and the louder it is telling your body to stop producing milk. 

The Moral of the Story

Between needing to produce oxytocin in order to inhibit the estrogen, produce prolactin to tell your brain to make milk, and drain milk to turn off the enzyme that's telling your body to not make any more, you need to nurse often.  If baby is under 9 months old, sleeps more than 4 hrs at night, and goes longer than 2 hrs during the day between nursings, you need to pay attention to supply.  You don't necessarily have to worry, but you need to pay attention.