Your Guide to the Menstrual Cycle

Medically Reviewed By Sanaz Ghazal, MD, FACOG
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From puberty until menopause, every month, a person with a uterus has a menstrual cycle resulting from hormone changes.

Sex and gender exist on spectrums. This article will use the terms “female” and “women” when discussing people assigned female at birth to reflect language that appears in source materials.

Learn more about the difference between sex and gender here.

Certain hormones prepare an egg from your ovaries for fertilization. The same hormones prepare your uterine lining for the implantation of an embryo, which is a fertilized egg. If an embryo does not implant, the lining of the uterus sheds during your menstrual period. The menstrual cycle then begins again.

This article will explain the menstrual cycle, what happens in each phase, and how to track it. It will also explain conditions related to the menstrual cycle and when to contact a doctor.

What is the menstrual cycle?

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Your menstrual cycle begins at puberty, around 12 years of age, but it can start between ages 8–15 years.

Menstruation ends at menopause, once you have had no periods for 12 months. The average age of menopause is 51 years, but it varies among individuals. Before menopause, your body prepares for pregnancy with every menstrual cycle.

Your menstrual cycle begins on the first day of your menstrual flow. This is when your body expels old uterine lining through your cervix and vagina. At the same time, your body begins to prepare another egg for fertilization. As your cycle progresses, your uterine lining builds up once again.

If your egg fertilizes, it will attach to this lining. If not, the lining will continue to shed through your cervix and vagina as your menstrual flow, or “period.”

Reproductive organs

The reproductive organs inside a female body consist of:

  • Two ovaries: At birth, your ovaries contain 1–2 million eggs.
  • Uterus: The uterus is where a fertilized egg implants and develops into a fetus.
  • Fallopian tubes: These tubes capture your eggs. When an egg fertilizes and becomes an embryo, the fallopian tubes transport it to your womb.
  • Cervix: This muscle is the opening of the uterus. It connects the uterus to the vagina.
  • Vagina: This canal leads to your cervix.  

Read more about women’s health and the female reproductive system here.

Length of the cycle

The menstrual cycle for most people lasts 28 days. Others can have a slightly shorter or longer cycle, ranging between 25–30 days.

Stress, weight loss, travel, medications, and certain medical conditions can cause fluctuations in hormone levels. These fluctuations can influence the length of your cycle from month to month.

What are the phases of the menstrual cycle?

Your menstrual cycle consists of 4 phases: menstruation, the follicular phase, ovulation, and the luteal phase.

Menstrual phase

If your egg does not fertilize and implant into the uterus, your estrogen and progesterone levels will drop during this phase.

If you do not become pregnant, your uterine lining is no longer necessary that month. It sheds through your vagina as your menstrual flow or period.

Follicular phase

The follicular phase starts the same day as your period starts. Follicle-stimulating hormones (FSH) and luteinizing hormones (LH) release from the pituitary gland. These hormones stimulate the growth in your ovaries.

Your eggs sit in the ovarian follicles, usually one egg per follicle. One follicle in the ovary becomes dominant and continues to produce estrogen.

Once estrogen reaches a certain level for some time, LH levels begin to surge. Ovulation occurs about 34–36 hours from the start of the LH surge and 10–12 hours after the peak.


In this phase, the dominant follicle in the ovary releases an egg, which the fallopian tubes capture. The egg waits in the fallopian tubes for sperm to fertilize it.

Luteal phase

Once the follicle releases an egg, the empty follicle develops into a corpus luteum structure. The corpus luteum begins secreting estrogen and progesterone. Increasing levels of progesterone cause the uterine lining to thicken and prepare for embryo implantation.

Once the egg releases, it travels into the fallopian tube, where the sperm fertilizes it. The fertilized egg, or embryo, travels through the fallopian tube and into the uterus, attaching to the lining.

The uterine lining breaks down and sheds if your egg does not fertilize and implant into the uterus. This is the beginning of your menstrual cycle.

What is a period?

When the thickened uterine lining does not need to support an embryo, it sheds as your period. Your period is a flow of about 3–5 tablespoons of blood, some mucus, and some tissue.

Period symptoms

When you have your period, you may feel the following symptoms:

How can you track your menstrual cycle?

There are many apps available that can help you track your cycle. Alternatively, you can use a simple calendar. Mark the first day you experience a full flow as the first day of your cycle. The length of your cycle measures from that day to the first day of your next period.

You can also track your ovulation by recording your basal body temperature or tracking changes in your cervical mucus.

The menstrual cycle and fertility

You can only become pregnant during your ovulation phase, which lasts about 24 hours.

Ovulation, or the release of an egg from the ovary, is necessary for you to get pregnant. An egg has about 12–24 hours after release for sperm to fertilize it before it dies.

A person’s fertile window refers to when intercourse is most likely to result in pregnancy. It is the 6 days leading up to and including the day of ovulation.

The timing of ovulation and the fertile window depends on the length of your cycle. For a 28-day cycle, ovulation will generally be on or around day 14. Therefore, the fertile window will begin around day 9 of your cycle.

If your cycle is longer or shorter, ovulation and your fertile window may vary. If you are having difficulty tracking this window, talk with your doctor.

What can cause an irregular period?

Up to one-third of people who menstruate will experience irregular periods at some point in their life. You are more likely to have irregular periods at the onset of both puberty and perimenopause.

Other reasons that your period may be late include:

  • stress
  • weight loss
  • travel
  • hormone imbalances
  • medications
  • pregnancy
  • recent pregnancy loss
  • medical conditions

Some medical conditions related to your menstrual cycle include:

  • Premenstrual syndrome (PMS): PMS can affect your mood, emotions, and behavior. Symptoms usually start about 7 days before your period and go away within 4 days of period onset.
  • Polycystic ovary syndrome (PCOS)PCOS is small, fluid-filled cysts developing in your ovaries. Symptoms include irregular menstruation, high levels of androgen or testosterone, other hormone imbalances, and insulin resistance, which can delay or prevent ovulation.
  • Fibroids: These are noncancerous growths in the uterus that can cause heavy bleeding.
  • Endometriosis: This is a condition in which tissue resembling the uterine lining grows outside the uterus. Endometriosis can cause irregular or heavy periods and pelvic pain, typically before and during your period.

Read more about PCOS, endometriosis, and fibroids here.

When to contact a doctor

Sometimes, irregularities or issues with your period may indicate an underlying problem. Contact your doctor if you experience any of the following:

Heavy bleeding

If you have period bleeding that lasts longer than 7 days, or if you need to change your pad or tampon nearly every hour, contact your doctor. This may indicate an issue with your menstrual cycle or reproductive system.

Irregular periods

Doctors consider menstrual cycles longer than 35 days or shorter than 21 days irregular. These cycles could indicate health issues.

Menstrual cycles longer than 38 days are common in the first 3 years after starting your period. When you enter perimenopause, your periods can also become irregular. However, if irregular cycles persist, talk with your doctor to rule out a health issue. 

Skipped periods

Hormone fluctuations can sometimes cause you to skip a period. You can wait a month to see if your period comes.

However, only menstruating every couple of months is unusual. If you go for several months without a period, talk with your doctor to understand why this may be happening.


Your menstrual cycle is a delicate interplay between many hormones. Knowing where you are in your cycle can make it easier for you to manage the changes.

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Medical Reviewer: Sanaz Ghazal, MD, FACOG
Last Review Date: 2022 Aug 11
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THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.
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