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The Science Behind Milk Coming in

After years of working in L&D, I’ve assisted many mamas with their baby’s first latch to the breast. Many first time mamas have questions or worries. Things like “I don’t think I have any milk” “How will I know if my baby is getting enough” “I never leaked in pregnancy, so I must not have any milk” are all common statements I hear. My usual response is something like “You’ve just had a baby, your body knows what to do. You need to trust your body to do its part if we do ours.”


But have you ever wondered about the science behind it all? How does someone start lactating? How does the body know to move from colostrum to milk and how does my body know what my baby needs and how much?

Let’s get down to the science behind lactation and your milk coming in. 


Breast tissue is full of mammary ducts and alvelus. The hormones produced during pregnancy encourages these ducts to grow to make space for the incoming milk. Think of a grapevine, the alveolus are the grapes that produce collect the milk, and the ducts are the vines that transport the milk to the nipple. Lactation is the result of a delicate balance of hormones, and stimulation of the nipple and breast tissue. 

There are five stages of Lactogenesis (science term for milk production from the mammary glands). We are going to focus on the first three which is the process of your milk ‘coming in’.

Lactogenesis I

is your colostrum. This milk is present from around 16 – 20 weeks of pregnancy, but you may not notice it until later in pregnancy other than your breasts getting bigger and heavier. This milk is readily available at birth, so your milk does not actually need to “come in”. Some women will be able to express this colostrum in the final weeks of pregnancy. But do not worry, if you do not leak colostrum, it does not mean that it is not there. And always speak to your healthcare provider prior to expressing colostrum in pregnancy. Colostrum is very important for baby. It has many protective properties to help keep baby healthy and protect baby’s gut.

Lactogenesis II

begins after birth. During pregnancy, abundant milk production is prevented by one of the many hormones produced by the placenta (this is why there are only small amounts of colostrum). Once the placenta is delivered after your baby, the placenta hormones drop dramatically and your autocrine system is now in charge of your lactation. With regular stimulation (aka feeding or pumping) Lactogenesis II typically occurs between 2 -3 days postpartum.

You will experience fullness in the breasts with the larger milk volume being produced and this is when most women feel that their milk has ‘come in’ even though it was there all the time. However maternal illness, traumatic birth, Gestational Diabetes, Cesarean Sections, retained placenta, or delayed early milk removal by a NICU admission may delay this phase of lactation. 

Other than the removal of the placenta, frequent and effective feeding is very important to initiate this second phase of lactation. Frequent and effective feeding or pumping stimulates the hormone Prolactin. Prolactin stimulates milk production, and Prolactin is produced directly in response to nipple stimulation – meaning breastfeeding, or pumping. This is called a Neurohormonal Feedback Pathway. The simpler way to explain it is Supply and Demand. The more you demand by pumping/feeding, the more lactation hormones are produced, and the more milk your body will supply your baby. 

Lactogenesis III

is the ‘stabilization’ of milk production and happens from around 9 days postpartum until the end of the breastfeeding journey. It is important to continue to have regular effective feeds during this phase of lactation. Your milk supply still relies on that Neurohormonal Feedback Pathway during this phase and will continue to do so until you cease to breastfeed or pump. During periods of growth, your baby will want to feed more. By following your baby’s cues the increased feeding sessions will increase your milk supply to meet their new needs.

Continuing effective feeds is important, as well as ensuring you empty the breast at every feed. If you do not empty your breasts fully, your breastmilk supply may decrease. This is due to FIL – Feedback Inhibitor of Lactation. It’s a small protein in your breastmilk, and the more FIL that is present in your mammary glands, the slower the milk production will be. By not emptying your breasts fully during each feed, you’re leaving milk behind with FIL that will signal your body to produce less.

Whoah, that was a lot of science, but trust me when I say this is still just the basics – I have 3 huge textbooks on the subject! The female body is so very cool when you look at what we can do when it comes to pregnancy and childbirth.

If you need support with the practical applications of Lactogenesis, please reach out to a local Mama Coach. We’d be happy to assist you with your lactation journey. 

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