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All you need to know about breast development! (In regards to lactation ☺️)

So we all learn a little something about breasts as young girls during sex ed, health class, our parents/friends, or on our own. We learn that during puberty, our hormone levels change which cause a slew of different reactions throughout our body. Our body hair changes, our odors change, our hips change, our fertility cycle begins and in turn, our breast tissue continues to develop. But this stage of breast development is not considered complete until pregnancy. Pre-pregnancy breast tissue is mostly fat tissue with some lactiferous units called lobes (Alex, et al, 2020).

During pregnancy your body goes through both anatomical and physiologic changes. It is very common for women to experience breast tenderness and growth in the first trimester. This is due to the influx of pregnancy hormones. "During the first trimester, the ductal system expands and branches out into the adipose tissue in response to the increase of estrogen. Elevated levels of estrogen also cause a decrease in adipose tissue and ductal proliferation and elongation. Estrogen also stimulates the pituitary gland which leads to elevated levels of prolactin" (Alex, et al, 2020). These hormones are preparing your body for lactation. These hormonal changes will occur with every pregnancy you have, therefore introducing more milk ducts the more pregnancies you go through.  

In the second trimester, it is most common for women to feel more like themselves and less like they are "pregnant". It is during this trimester of pregnancy that "by the twentieth week of gestation, mammary glands are sufficiently developed to produce components of milk due to prolactin stimulation. Milk production is inhibited by high estrogen and progesterone levels and colostrum is produced during this time" (Alex, et al, 2020). Some women see this change by leaking milk colostrum. Or if you are nursing a child from a previous pregnancy during your pregnancy, your child will start drinking colostrum again. This could help them wean if they do not like the taste of the change, or they may not care about the change at all. Either way, if you are nursing during pregnancy (which you should have spoken to your OBGYN about at the beginning of pregnancy to discuss if it is safe for you) this is totally fine for your child to consume. Colostrum is still breastmilk, just a different nutritional make up.

Now on to the third trimester! Not too many changes occur here, but your hormone levels slowly start decreasing. Which brings us to birth! After the birth of your baby, you birth the placenta. The placenta holds a lot of your hormones, so once birthed, those hormone levels immediately plummet. "In the third trimester and then rapidly after birth, these levels decrease, allowing for milk production and eventual let-down to allow for breastfeeding" (Alex, et al, 2020). Because this is a process and your hormone shifts take some time to show themselves outwardly, the first 3-5 days after birth, your milk still has the nutritional make up of colostrum. But by day 3-5 your hormones levels have shifted enough where your milk has now transitioned over to "mature milk". This is the nutritional make up needed to sustain your newborn (though this nutritional make up will constantly be changing throughout your entire nursing journey as your body consistently adjusts to your baby's ever changing needs).

Nursing is a two way street; your baby removes your milk telling your body to make more and your baby's saliva will also permeate through your skin/glands on your nipple/aereola which your body will then regulate to. For example, if baby is sick, this will show in their saliva. Your body will notice those germs and react to them, telling your body to send more white blood cells and bacteria fighting components to help your baby fight off whatever kind of illness they may have. Therefore, if you consistently nurse your baby, your breastmilk becomes 100% custom to their biological needs. This is truly the most magical thing your body will ever be able to do, in my opinion ☺️!

Finally, as your nursing journey comes to an end, your hormones continue to level out back to their pre-pregnancy state. But this cessation actually takes much longer than you may think. This is usually because once you get your period back (this timeframe is very different per individual per pregnancy), your breastmilk production will then fluctuate with your cycle.

Your body cannot prepare for lactation until your hormone levels reach a certain point and then it needs to reach the next point to create milk and then sustain it. So it seems reasonable that when your period returns, your supply will run the same course as your hormones roller coaster through your cycle. Most women who have their period while still nursing notice their supply will dip each month while they bleed and it will pop back up to its baseline once their bleeding stops.

Which then leads to the cessation of lactation. This can only be considered complete, 40 days after you stop feeding. "40 days post cessation of lactation - Post-lactational involution occurs at the cessation of milk production caused by a decline in prolactin" (Alex, et al, 2020). Your body must go through a full cycle of hormone levels to go back to its original state of pre-pregnancy levels.

I hope after reading this blog, you have noticed and understand the correlation between hormones and lactation. Hormones are what control the entire process of lactation. Without them, you cannot lactate. Therefore, without going through the the first half of pregnancy (with all those hormone shifts), your breasts will never go all the way through to completely develop.

Please come back soon for another Hungry Baby Reflection! I have so many other topics I would love to talk about with you all! Thank you for reading!


Alex A, Bhandary E, McGuire KP. Anatomy and Physiology of the Breast during Pregnancy and Lactation. Adv Exp Med Biol. 2020;1252:3-7. doi: 10.1007/978-3-030-41596-9_1. PMID: 32816256.

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