What Happens to the Body During Each Phase of the Menstrual Cycle?
Key takeaways:
The menstrual cycle typically lasts between 21 and 35 days, though it is normal for it to be shorter or longer for some people.
During the cycle, different hormone levels increase and decrease, leading to changes in the uterus and ovaries to prepare for ovulation and possible fertilization of the egg.
If the ovulated egg is not fertilized, the hormone levels decrease and the uterine lining is shed, leading to menstruation and the beginning of a new cycle.
One menstrual cycle is measured from the first day of someone’s period until the first day of the next period. On average, this cycle lasts 28 days, but it can be normal to last between about 21 and 35 days.
The menstrual cycle is broken up into different phases. These phases are defined by the changing levels of hormones in the body. These hormones help prepare the uterus and egg that is maturing in the ovaries for possible fertilization and pregnancy.
We’ll explain these different phases and what happens in the body during the normal menstrual cycle.
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Which hormones are important in the menstrual cycle?
Many hormones are involved in the menstrual cycle. But four of them are especially important:
1) Follicle stimulating hormone (FSH) stimulates ovarian follicles to grow. A follicle is a small sac that contains a developing egg. During each menstrual cycle, many follicles start to mature and prepare for ovulation (the release of an egg). These follicles compete with each other. In the end, only one follicle usually releases a mature egg during ovulation.
2) Luteinizing hormone (LH) peaks right before ovulation and tells the dominant follicle to release an egg.
3) Estrogen is produced by the growing follicles in the ovaries. It signals the uterine lining to grow and prepare for a potential egg fertilization.
4) Progesterone is also produced by the ovaries. It helps to keep the uterine lining ready for possible embryo implantation.
The start of the menstrual period: The follicular phase
The menstrual cycle begins when the uterus sheds its lining from the prior cycle. This first day of bleeding is considered Day One of the menstrual cycle. And this is the start of the follicular phase. Most menstrual bleeding lasts from 4 to 7 days, though this can naturally vary from person to person.
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Hormone levels are at their lowest at Day One. During the follicular phase, the FSH level begins to rise and stimulate follicles in the ovaries to grow. As the follicles grow, they release estrogen. The rising estrogen levels then stimulate the uterine lining to grow.
As this phase progresses, one follicle becomes dominant over the others. This is the one that will eventually release an egg, an event known as ovulation. Ovulation is triggered by a peak in LH, which occurs at the end of the follicular phase (about halfway through the cycle).
The midpoint: Ovulation and the start of the luteal phase
After the LH peaks and ovulation occurs, the luteal phase begins. It tends to happen around Day Fourteen. But the day of ovulation can differ from cycle to cycle and from person to person.
After releasing an egg, the follicle’s job is not done. It becomes what is called a corpus luteum, which produces progesterone and estrogen. These hormones work together to grow and maintain the lining in the uterus. This prepares the uterus to accept a potential embryo if the egg is fertilized.
An unfertilized egg survives about 12 to 24 hours after being released from the follicle. During this time, it travels down the fallopian tube towards the uterus. It will be fertilized only if it encounters sperm during this journey. Sperm can survive for up to 5 days after ejaculation. So these sperm may be present from before or after ovulation.
The time from ovulation to menstruation (and a new cycle) generally lasts about 14 days. This phase is usually more consistent in duration than the follicular phase.
The end of the luteal phase
If the egg is not fertilized, the corpus luteum deteriorates. When this happens, it stops making hormones, so progesterone and estrogen levels begin to fall. Once these hormones reach a low level, the uterine lining is no longer receiving the signal to stay intact. And so it will begin to shed, thus beginning another menstrual cycle.
Are menstrual cycle phases the same for everyone?
There is some expected variation in the menstrual cycle in different people — and even between cycles in the same person. Many people can fall outside of the “normal” range of cycle length and still be perfectly healthy. One study of over 600,000 menstrual cycles found significant variation in cycle length, day of ovulation, length of each phase, and length of the menstrual period.
If you feel that your cycle has changed from what is normal for you, your healthcare provider can help. They can measure your hormone levels during different times in the cycle to help determine if there is a problem.
The bottom line
It can be helpful to understand what is happening in our bodies as we cycle through menstruation. When a menstrual cycle starts, hormone levels begin to rise to prepare the developing eggs and uterine lining for a potential pregnancy. If the egg is not fertilized, these hormone levels fall, the uterine lining is shed, and a new cycle begins.
There are expected differences in the menstrual cycle, and it can even differ in the same person between cycles. These differences are often healthy and normal.
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References
Bull, J. R., et al. (2019). Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digital Medicine.
Office on Women’s Health. (2018). Menstrual cycle.
Office on Women’s Health. (2018). Your menstrual cycle.
Pillarisetty, O. R., et al. (2021). Anatomy, abdomen and pelvis, ovary corpus luteum. StatPearls.
