How Your Breasts Change Throughout Your Cycle

Surprise! Your uterus isn’t the only part of your body that’s working hard, round-the-clock.

Your breasts are constantly growing and changing, too.

Throughout your menstrual cycle, your breasts, alike the many other systems in your body, are changing to accommodate the hormonal fluctuations that each phase of your cycle brings.

To understand the developments that are occurring, we must first understand the menstrual cycle, as a whole.

Your Cycle as a Whole

Every month, your body works clockwise through the four phases of the menstrual cycle.


Menstruation marks the beginning of your cycle. Here, the uterus is shedding the lining built during the follicular/ovulatory period, as well as expelling the excess fluid (blood) that it had been holding.


This phase begins along with your period, and prepares the body for ovulation.


Ovulation is the sole purpose of your cycle.  During this phase, one of your ovaries (your body alternates between the two) will send an egg to the your uterus, where it will then stay until it is either fertilized, or expelled during your next menstrual bleed. As preparation for this process, our uterine lining will thicken, and our uterus will begin to withhold more fluid (blood) than usual.


This phase occurs after ovulation. The two outcomes of this phase are either pregnancy, or continued menstruation.

Further information on the Follicular and Luteal phases can be found here.

How They Change

As you can see, your body has a lot going on, all the time. However, it is widely unknown that outside of the uterus, many other bodily systems go through monthly changes as well. Breast growth and development (which is typically overlooked as just “boob pain”) is not only a PMS symptom, but an individual part of the menstrual cycle.

The female chest muscle system.

Here’s how your breasts change throughout your cycle.


Within the first two days of menstruation, your breasts have already begun the building and development of cells and tissues. Passageways for breastmilk, and milk-ducts themselves, are being formed, and the growth of pre-existing ducts and lobes continues.

Breast anatomy via the National Breast Cancer Foundation. Pictured are the ducts, in which breastmilk travels through, and the lobes, where it is stored.

This rapid growth is caused by the rise in two hormones: estrogen, and progesterone. During the luteal phase (the phase before your bleed), the body is triggered by these hormones to prepare for pregnancy. Whether you’re actually pregnant or not, your body still goes through the motions of getting ready for a baby. This includes holding extra fluid (blood) in the uterus, and preparing the breasts for milk-production.

Externally, the forthcoming hormone rise can cause the breasts to look bigger, and “weirder.” The lumpy, tight feelings you experience are simply caused by the internal expansion you are undergoing. Luckily, this is only temporary, as the body is coming down off of the ovulatory/luteal hormones (or baby high), and your breasts will soon return to their natural shape and size.

That being said, the first few days of your period are still being affected by the hormones released during the luteal phase (hence the expansion of milk ducts and passageways). But towards the end of your period, these hormones will begin to settle down.


Throughout the last few days of your period, and the beginning of your follicular phase, the breasts may appear smaller, uneven, or “deflated.” This is caused by the drop in estrogen and progesterone in the later days of your period. But once the follicular phase truly begins, 4-5 days after your menstrual bleed, your breasts will begin to level out again.

Estrogen levels will rise at the peak of the follicular phase. Progesterone will return later in the ovulatory phase, but for now, estrogen is the most impactful hormone. Estrogen is a uniquely feminine hormone, influencing both the body, and mind.

The duty of the follicular phase is to prepare the body for ovulation. During this phase, the uterus is working hard to build a thick lining, and retain fluid to create a nourishing environment for the egg. Your breasts, and other bodily systems alike, have a lot of work to do too. Estrogen, as stated before, will be a great helper during this preparatory period.

Towards the end of your follicular phase, and the early days of ovulation, estrogen plays an important role. This hormone is what kickstarts the development within the breasts (growth of breastmilk ducts and passageways). The rise in estrogen will also cause them to be more appealing appearance wise, as well as fuller, firmer, and higher on the chest. A win-win for you, and your body.


Finally, the purpose of it all! If your cycle was the solar system, ovulation would be the sun — all your body’s work (your work!) revolves around this.

During the ovulatory phase, estrogen and progesterone get down to business. An egg is released by one of your ovaries, and will make its way to the uterus, where it will remain nice and cozy until it is either fertilized and developed, or expelled in your next period. See, all that talk of thick linings and excess fluid finally makes sense! The environment in which the egg resides, for however long or short, is extremely important.

As far as breast development goes, not too much changes from where we left off in the follicular phase. Generally, your boobs always look great when you’re ovulating. It’s a fact! Internally, they are getting ready to (you guessed it) expand the breastmilk production system. This may cause some tenderness or swelling, but all in all, not too much will change.


As we learned in the menstruation phase, the luteal phase sparks some amazing internal endurance! During this phase, the uterus is working hard to maintain its thick lining and retain all the of the excess fluid it has been holding. Passageways for breastmilk, and milk ducts themselves, are being expanded and developed.

During the luteal phase, levels of estrogen, and more significantly, progesterone, are on the rise. Some of the most classic PMS symptoms — such as breast pain and swelling — are actually caused by the changes that occur during the luteal phase.

These two hormones — estrogen, the female hormone, and progesterone, the pregnancy hormone — are what make it all happen. The continuation of the nourishing uterus, and the development of the breastmilk system, are all done to preserve an egg, and prepare it and the body for pregnancy. Regardless of whether the egg is fertilized or capable of developing, this process will still occur.

This preservation of the uterus and development of the breasts will continue until the period begins, but some expansion of the breasts will continue for a few days after, or until the hormone levels decrease.

What to Take From This

Every person with breasts, at some point in their life, is insecure about them. Knowing your body, and all that it does for you, is one of the most impactful things you may learn. Knowing the real reason why you are changing everyday, and why you don’t always look “right,” is the key to alleviating the anxiety surrounding your appearance, and in turn, accepting it.

And, if nothing else, boob science is really cool!

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