This is what the inside of your breasts look like. Your alveoli is where your milk is made and stored between feeds. Alveoli are tiny sacs grouped together like little bunches of grapes. They’re made of a type of cell called ‘lactocytes’, which make milk. Laying close to these milk-making cells are blood vessels which provide everything you need to make milk for your baby. In between the lactocytes are lymphocytes, cells which produce antibodies for your milk to help your baby fight off infection. Each alveoli is surrounded by cells which contract when you have a let down to squeeze the milk through your ducts and out of the openings in your nipple (1).
Boobs come in all shapes and sizes. They are primarily made up of fat tissue and glandular tissue (the parts which make and transport your milk). We all have a different amount of glandular tissue and a different amount of fat tissue, and the proportions between the two differ greatly from woman to woman. Large or small, mainly glandular tissue or mainly fat tissue, over 95% of us are able to make enough milk to feed at least one baby. It is not the amount of glandular tissue which determines your milk supply, but rather your baby’s appetite (2). This is explained in more detail below.
Lactogenesis is the process of your breasts beginning to make milk. It is split into three stages:
- Lactogenesis I: Prolactin is a hormone which tells the milk-making cells in your breasts to produce milk. It is also the hormone which is responsible for some of your maternal instincts, such as nesting and the urge to protect your baby. During pregnancy, your prolactin levels rise to about 20x the normal level in preparation to feed your baby. You begin making your first milk, colostrum, when you’re about 16 weeks pregnant, but the hormone progesterone is being released in high volumes by your placenta, and this inhibits prolactin and stops you making too much milk before your baby is born. Some mums notice a bit of leaking during pregnancy, but don’t worry if you don’t, it’s still there! In the first few days after birth, this small amount of colostrum is all your baby needs.
- Lactogenesis II: After you birth your placenta, your level of progesterone begins to fall. By about 3-5 days after your baby is born, your progesterone level is low and prolactin is no longer inhibited. This makes your breasts go into overdrive and produce copious amounts of milk. This is often referred to as your milk ‘coming in’, though as explained above, you are already making milk before this happens! This is all hormonally controlled, so as long as the right hormones are in place this will happen whether you are breastfeeding or not.
- Lactogenesis III: From about 8-14 days after the birth of your baby, until the day your baby weans, you are making mature milk. Now your milk is produced on a supply and demand basis (3), as explained below.
How milk supply is maintained
The amount of milk you make is determined by your baby’s appetite. The more milk you take out of your breasts, the more you produce. Everytime you feed your baby your brain releases a pulse of prolactin, which tells your breasts to make more milk. After 6 weeks or so when your milk supply is established, it is largely maintained by a protein called the ‘Feedback Inhibitor of Lactation’ (FIL). FIL is a whey protein in your milk which tells your breasts to slow down milk synthesis when they start getting full. When there is lots of milk in your breasts, there is lots of FIL and so production slows down. When your baby feeds, they remove some of the milk and some of the FIL along with it, so production speeds up. The emptier your breasts are, the faster they are producing milk, and the more often milk is removed, the more often they are filled up (4).
Your breasts are never empty. Trying to empty your breast would be like trying to empty a glass of water by drinking it through a straw whilst you’re holding it under a running tap. No matter how much you drink, the glass will never be empty, because it is always being refilled.
Your let down reflex
When you breastfeed, your brain releases a rush of oxytocin which causes the alveoli in your breasts to contract, pushing your milk out through your ducts. This is known as the ‘let down’ or ‘milk-ejection’ reflex. A let down is usually triggered by nipple stimulation when your baby feeds, but it can also be triggered just by hearing a baby cry, looking at a baby, by the oxytocin released during sex, or even just randomly! You may experience a tingling or stinging sensation in your nipples, and will notice your baby’s sucking pattern changing from rapid sucks to slow deep sucks and swallows. Oxytocin also causes the blood vessels on your chest to dilate, which helps to regulate your baby’s temperature, and it is also the hormone which creates feelings of love and makes you yearn to respond to your baby (5).
Colostrum is the first milk that your breasts produce, beginning at around 16 weeks of pregnancy. You may experience leaking or may be able to express a little colostrum whilst you are still pregnant, or you may not see any colostrum until after your baby is born. Colostrum is produced in very small amounts. Newborn babies’ stomachs are very small so they only need a very small amount of breastmilk at each feed. Colostrum is higher in proteins, minerals, fat-soluble vitamins and anti-infective factors than mature milk, providing your baby with a perfect balance of nutrients and passive immunity. It also contains high concentrations of Beta-endorphin, a protein which binds to opoid receptors in the brain and reduces pain, to help your baby recover from birth. It has a laxative effect, helping your baby to pass their meconium, promotes the growth of good bacteria in the gut, and helps to clear the excess bilirubin that causes newborn jaundice (3).
Around 8-14 days after birth, your breasts are making mature milk. This milk is highly variable in colour, consistency, and both nutritive and non-nutritive components to continually meet your baby’s changing needs. For example, you will produce milk with higher water content in hot weather, and milk containing more anti-infective properties when you or your baby are baby ill (3). See here for some of the amazing properties in your milk.
(1) Physiology in Childbearing with Related Biosciences- Jean Rankin (4th edition, 2017)
(2) Goren A et al. Journal of Biological Homeostatic Agents. 2017 Jan-march  Ramsey DT, Kent J.C, Hartmann R.A, Hartmann P.E, Anatomy of the lactating human breast refined with ultrasound imaging, Journal of Anatomy, 2005 Jun; 206(6): 525-34
(3) Counselling the Nursing Mother- Judith Lauwers and Anna Swisher (6th edition, 2016)
(5) The Oxytocin Factor: tapping the hormone of calm, love and healing, Kerstin Uvnas Moberg, foreword by Michel Odent (2011)