While most women are familiar with the general patterns of their menstrual cycle, it never hurts to get a refresher on what is actually going inside your body during your cycle. Here are the key points to remember.
The normal menstrual cycle is a tightly coordinated cycle that results in the release of a single mature egg from a pool of hundreds of thousands of immature eggs.
The first day of your period marks the first day of your menstrual cycle. The usual duration of menstrual flow is 3-5 days, but can last as little as 1 day in some women, or 7 days in others.
The average adult menstrual cycle lasts 28 to 35 days (yes, the length of the cycle is notoriously variable in women!) In general, menstrual cycle length peaks at approximately age 25-30 years, and gradually declines, so that women in their forties have slightly shorter cycles.
The cycle is divided into two phases.
The follicular phase begins on the first day of your period and ends on the day before the luteinizing hormone (LH) surge (we’ll learn more about the LH surge below). This phase lasts approximately 14 to 21 days (variability of your menstrual cycle duration is tied to the length of your follicular phase).
The luteal phase begins on the day of the LH surge and ends at the start of your next period. This phase lasts 14 days.
FSH is a hormone that stimulates your ovaries to produce steroid hormones (that is- estradiol during the follicular phase and progesterone during the luteal phase). LH is a hormone that also stimulates the ovaries to produce steroid hormones, and its surge at the middle of your cycle provokes ovulation.
The follicular phase begins on the first day of your period.
At the start of the follicular phase, your ovary is the least hormonally active (low estradiol and progesterone concentrations). Progressively, a small increase in FSH secretion will recruit the next group of developing follicles (fluid-filled sacs that contain an immature egg), one of which will become dominant and ultimately release the egg during that cycle.
As these follicles grow, they start producing increasing amounts of estradiol. The increasing estradiol concentrations result in the proliferation of the uterine endometrium (innermost lining of your uterus), which becomes thicker.
As estradiol increases daily during the week before ovulation (so approximately between days 7 and 14 after the start of your period), FSH and LH concentrations fall (they’re inhibited by estradiol). One dominant follicle is selected, and the rest of the growing follicles gradually stop developing and degenerate. Rising estradiol continues to thicken the uterine endometrium, and causes an increase in the amount of cervical mucus (which you might be able to detect).
The luteal phase is the second half of your menstrual cycle: it starts after ovulation and ends with the first day of your period.
Approximately one day before ovulation, serum estradiol concentrations peak (this is called the midcycle surge). At this point, estradiol strongly stimulates LH secretion. This “LH surge” causes the maturation of the egg inside the dominant follicle, and eventually its release from the surface of the ovary. The egg then travels down the fallopian tube to the uterine cavity. The cells that previously surrounded the egg start to produce the hormone progesterone, which continues to thicken the lining of your endometrium so that a fertilized egg can implant.
If the oocyte does become fertilized, it implants in the endometrium several days after ovulation.
In the absence of fertilization, a decrease in LH secretion results in a gradual fall in progesterone and estradiol production. This results in the loss of endometrial blood supply, shedding of the endometrium, and the onset of your period (approximately 14 days after the LH surge). FSH levels then start to rise again, thereby beginning the next menstrual cycle.
Remember that every woman’s menstrual cycle is unique (in cycle length and flow).
It’s important for you to be familiar with your cycle, in particular when you get your periods, how long they last and how heavy your flow is. Pay attention to any changes, and report them to your healthcare provider.
This diagram may help with visualizing the cycle progression:
Resources:
https://www-uptodate-com.ezproxy.med.nyu.edu/contents/physiology-of-the-normal-menstrual-cycle.
