The Four Phases of Your Menstrual Cycle
Female bodies are cyclical by nature, producing varying quantities of sex hormones at different points in our monthly menstrual cycle—namely, estrogen, progesterone, and testosterone.
Other hormones, such as follicle-stimulating hormone (FSH), luteinizing hormone (LH), anti-Müllerian hormone (AMH), prolactin , oxytocin , and cortisol, as well as neurotransmitters such as serotonin, epinephrine, and dopamine, also fluctuate during this cycle.
These hormones are all directly and indirectly responsible for four distinct phases of the menstrual cycle. Yes, our bodies are constantly ebbing and flowing, and each phase of the monthly cycle brings about significant physical and emotional changes.
For women and menstruators who are getting their period roughly every 25 to 35 days, this occurs on an almost weekly basis. Yes, that is correct, we are basically different people every week of our cycles. But contrary to what society says, this is not a bad thing! For those with longer cycles, the follicular phase will be longer and for those with shorter cycles, the follicular and/or the luteal phases could be shorter.
Mastering our internal rhythms is the key to feeling more connected to our bodies and generally, the first step to solving a lot of the physical and emotional complaints associated with our cycles. This knowledge will help prepare you for your own natural ebbs and flows and will provide important clues about whether your hormones are functioning the way they are supposed to in the various phases.
Phase 1: The Bleeding Phase (Menstruation)
Let’s start with phase 1, the bleeding phase, also known as menstruation. Day 1 of your menstrual cycle is the first day of bleeding. On average, the bleeding phase lasts about 3 to 7 days. Right before your period, progesterone levels plunge, causing the breakdown and shedding of the uterine lining. As menstruation gets underway, estrogen and progesterone are at the lowest they will be in the entire cycle, with progesterone staying low until ovulation occurs.
The cervix, the cylindrical-shaped tissue that connects the vagina and uterus, changes position throughout the cycle. In this first phase, it is firm to the touch, sort of like the tip of your nose; in a low position; and slightly open to allow menstrual blood to pass through.
The bleeding phase also marks the first half of the follicular phase - I call it the menstruation part of the follicular phase. At this point, the region of the brain known as the hypothalamus has already been secreting gonadotropin-releasing hormone (GnRH) since a few days before the end of your last cycle. Remember from the last lecture, GnRH instructs the pituitary gland (the “master endocrine gland”) to release follicle-stimulating hormone or FSH.
The FSH has been communicating with the ovaries to start recruitment of a handful of ovarian follicles, each of which is a little balloon-like sac that contains a single egg.
So, between days 1 and 4, while we are on our periods, FSH continues to stimulate the follicles.
If you are testing your FSH, this is why it would be tested between days 2 and 4, to see whether it is at the appropriate level to be doing the all important job of stimulating the follicles on the ovary. This is the baseline level of FSH. Additionally, estrogen is supposed to be low at this time in the cycle, which helps get an accurate picture of FSH levels because higher estrogen will artificially suppress FSH.
Between days 5 and 7, just as menstruation is wrapping up for most of us, one follicle from the selected group is chosen. The rest of the follicles in the group will disintegrate.
Phase 2: The Follicular Phase
Now we’re moving into the non-menstruation half of the follicular phase, which is the time in the menstrual cycle when the ovaries continue preparing for ovulation.
It’s the length of the follicular phase that varies and will determine how long an actual menstrual cycle is. Once you ovulate, you’ll have a pretty good idea of when your period is coming.
Cells on the outside of the follicles produce androgens. These androgens are then converted into estradiol by the granulosa cells, which are on the inside of the follicle. This happens because FSH activates something called the aromatase enzyme in the granulosa cells, which converts these androgens to estrogens.
This is how the maturing follicles produce estradiol in increasing amounts. These higher levels of estradiol send a signal back to the hypothalamus to say all is well in ovary land, and the follicles are growing as they should.
As estradiol continues its ascent, it signals the hypothalamus to tell the pituitary gland to slow down FSH production and crank up luteinizing hormone production. Rising LH stimulates production of androstenedione and testosterone. Androstenedione will be converted to estrone and then estradiol. This ensures a steady supply of estrogen so that there is enough to trigger ovulation.
High prolactin levels actually decrease androgen conversion to estrogen in the ovary, causing higher androgens and lower estrogen. This becomes problematic because without a high enough level of estrogen, there won’t be the LH surge which kicks off ovulation. This is the mechanism by which elevated prolactin would prevent ovulation from occurring.
Around day 8 of the menstrual cycle, or halfway through the follicular phase, that chosen follicle dominates.
Now let’s talk about what is happening with cervical fluid during all of this hormone hullabaloo. As ovulation approaches, estrogen also prepares the uterus for pregnancy, thickening the blood vessels of the uterine lining. The cervix gradually moves higher in the vaginal canal and opens. And as for cervical fluid, after your period, you may notice little to no cervical fluid and a “dry” vaginal sensation. In other words, when you touch your vulva it feels only slightly moist, and when you wipe yourself it feels dry.
However, as estrogen builds during the follicular phase and stimulates the cervical crypts, you’ll start to see more cervical fluid and it begins to take on a wetter consistency, often looking pasty, creamy, or like lotion initially. And then as you approach ovulation it will change and become even more wet. You are most fertile in the second half of the follicular phase leading up to ovulation, and a barrier method of birth control should be used during this time if you are not planning to get pregnant. Consequently, this would be the time to have sex if you’re wanting to get pregnant. After ovulation, progesterone will take over and will make cervical fluid more sticky or tacky or even dry it up for the most part.
Phase 3: The Ovulatory Phase
Contrary to popular belief, ovulation, not menstruation, is the star of the menstrual cycle show. The ovulatory phase is the shortest phase, but it’s the one that packs the biggest hormonal punch, because ovulation is the driver of all the hormone production throughout the cycle. Essentially, it is the culmination of all the hard work your body has been doing throughout the follicular phase.
Estradiol levels continue to rise in parallel to the size of the maturing dominant follicle and the increasing number cells. A dramatic rise in estradiol from the maturing follicle tells the hypothalamus to trigger the mid-cycle LH surge from the pituitary that is needed to initiate ovulation.
An interesting fact. In order for the level of estradiol to reach the threshold that is needed to initiate the LH surge, the dominant follicle is almost always over 15mm in diameter when measured on ultrasound. Additionally, estradiol levels must be greater than 200pg/mL for approximately 50 hours in order for the LH surge to occur. These can be helpful pieces of information for those who are trying to get pregnant.
The LH surge starts about 35-44 hours prior to ovulation, and it typically occurs between midnight and 8am.
The start of the LH surge can be a good predictor for when ovulation will take place. However, it doesn’t mean ovulation will actually happen. There are times when LH rises but never reaches its peak, so ovulation doesn’t occur and then the body will likely try again to ovulate.
The LH surge will induce luteinization cells in the ovary, which, put simply, leads to the production of progesterone and ultimately, the formation of the corpus luteum once the egg is released.
Once an egg is released, it is viable for twelve to twenty-four hours. At this point, your cervix becomes soft, it moves up higher in the vaginal canal, and opens. In preparation for the sperm, your cervical fluid transforms into what is known as fertile-quality cervical fluid, becoming clear (aka translucent) and viscous (think raw egg white) and highly elastic or very wet and watery.
Phase 4: The Luteal Phase
Now we’re moving onto the luteal phase. This phase typically ranges from 11 to 17 days but is about 12 to 14 days in most women. When the luteal phase fits into this range, it’s considered to be a fertile cycle. The luteal phase is dominated by the hormone progesterone.
The length of the luteal phase is based entirely on how long the corpus luteum (the follicle that released the egg) maintains its progesterone production.
There is a condition known as Luteal Phase Deficiency, which is when a luteal phase is less than 9 days long. It is characterized by insufficient progesterone exposure to maintain the endometrium and allow for implantation of an embryo.
Generally speaking, a luteal phase needs to be at least 10 days long in order for that particular cycle to be considered a fertile cycle. This is because the egg (if it has been fertilized) needs a certain amount of time to travel down the fallopian tube and implant into the progesterone-prepared endometrium. If the endometrium starts shedding before it gets there, or if there isn’t enough progesterone to make it receptive to the fertilized egg, then there will be no pregnancy.
After ovulation, FSH and LH levels decline, with LH remaining low for the rest of the cycle and FSH rising slightly before menstruation to get the next round of follicles ready. Estrogen continues its sharp decline, while progesterone continues its climb thanks to the corpus luteum’s progesterone output. Progesterone will stay high throughout the luteal phase.
Progesterone is a thermogenic, or heat-inducing, hormone, meaning it raises basal body temperature for the remainder of the luteal phase. This rise in temperature is an important indicator of whether you’ve ovulated. Progesterone also further prepares the endometrium for a possible pregnancy. It transforms cervical fluid from stretchy and wet to opaque and sticky, or less fluidlike. If examined under a microscope, it would appear to have a basket-weave texture, which serves as your vagina’s very own sperm barrier. This is why it is referred to as infertile cervical fluid, as it’s particularly hard for sperm to swim through at this stage.
During the second week of the luteal phase, estrogen makes one more appearance, in a last-ditch effort to further prepare the endometrium for pregnancy. Due to the higher estrogen, you may notice an increase in cervical fluid resembling that seen in the lead-up to ovulation. Keep in mind, this is not fertile-quality cervical fluid, so you can’t get pregnant during this time.
If the egg is fertilized, it starts producing human chorionic gonadotropin (hCG) hormone and continues to make its way down the fallopian tube to the uterus. The hCG will signal the corpus luteum to keep making progesterone as well as estrogen, to support a pregnancy in its early stages. The corpus luteum’s production of progesterone will support a pregnancy for approximately nine weeks before the placenta begins taking over fully. This usually happens somewhere between 7-10 weeks after conception.
If there is no pregnancy, the corpus luteum function begins to decline about 9 to 11 days after ovulation. But as I said before, the luteal phase can be as long as 17 days, although this rarely happens. The drop in estrogen and progesterone that follows the corpus luteum breakdown will tell the uterine lining it’s time to go, and then you will get your period.
Our bodies are so amazing!