Menstruation: what is a period, when does it start, and what’s normal?

last updated: Sep 03, 2021

5 min read

Having a regular menstrual period is typically considered a marker of normal female reproductive health and wellness. Since it’s a monthly occurrence experienced by a majority of women, you might take your body’s actions for granted. 

However, your period is actually the result of interactions between very complex systems in your body. This involves your reproductive organs, hormones, and even structures in your brain.

Let's take a look at the basics of menstruation: what it is, when it starts, and what’s considered normal.

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What is menstruation?

The inner lining of the uterus is called the endometrium. Starting at around 12 years old on average, your endometrium begins to grow thicker each month in response to changing hormone levels in the body. This prepares it for receiving a fertilized egg. If no pregnancy occurs that month, the uterine lining flows out of the body. This is your monthly menstruation period or menses (Lacroix, 2021).

Menstruation occurs approximately every 28 days, although it can range from every 21 days to every 45 days. Your period will last between three to seven days on average. Menstruation that lasts longer than ten days is considered abnormal (Lacroix, 2021).

Regular menstruation relies on many structures and processes in the body functioning normally. Any interruption in one of these can make menstruation irregular, including (Lacroix, 2021):

  • The hypothalamus and pituitary glands in the brain

  • The ovaries, fallopian tubes, uterus, and vagina

  • Adequate nutrition 

  • Absence of chronic illnesses

What’s in a period?

The menstrual blood that you lose during your period isn’t actually made up entirely of blood. It also contains endometrial tissue debris and prostaglandins, a type of lipid that has hormone-like effects. There is some blood, though, usually between 30–80 mL (about 1–2.5 tablespoons). More than this would be considered heavy menstrual bleeding. Menstrual blood does not usually contain clots unless the flow is very heavy (Thiyagarajan, 2020).

Several factors can affect the amount of flow that you experience during your period. These include (Thiyagarajan, 2020):

  • Medications you take

  • Your hormone levels

  • The thickness of your endometrium

  • Blood disorders

  • Clotting disorders

In the first two to three years after you start menstruating, it's common for your cycles to be irregular. Young teenagers might have several months of missed or irregular periods. Eventually, hormone interactions balance out, and your cycles become more regular (Lacroix, 2021).

Typically, regular menstruation will continue until you reach menopause, although it will stop or be extremely light while pregnant or breastfeeding. Menopause typically begins at around 51–52 years and is caused by a drop in estrogen levels. Very few women (about 5%) may experience menopause as early as 40 years old (Peacock, 2021).

What is the menstrual cycle?

The first day of your menstrual period is considered day 0 of a normal menstrual cycle. The menstrual cycle is the monthly hormonal rhythm a female's body goes through to prepare for pregnancy. It includes natural changes in hormone production and the structures of the uterus and ovaries. This cycle is divided into phases (Thiyagarajan, 2020).

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The follicular or proliferative phase

This first phase of the menstrual cycle occurs from day 0 (the first day of your period) to day 14 based on the average duration of 28 days. The menstrual cycles vary in length due to this phase (Thiyagarajan, 2020). 

The follicular phase’s purpose is to grow the endometrial layer of the uterus and get ready for possible fertilization. Estrogen is the primary hormone that regulates this phase (Thiyagarajan, 2020). 

This phase is essential for creating an environment that’s friendly to possible incoming sperm. Estrogen creates channels within the mucus of your cervix, which will allow sperm to enter. At the same time, a follicle in your ovary that contains an immature egg starts to mature (Thiyagarajan, 2020).


Ovulation, or the release of an egg, always occurs 14 days before your next period starts. In an average 28 day cycle, this means it will happen on day 14 (Thiyagarajan, 2020).

At the end of the follicular phase, estrogen levels are at their highest. This also encourages follicle-stimulating hormone (FSH) and luteinizing hormone to increase. These high levels of hormone cause a mature follicle in the ovary to rupture and release an egg.  At the same time, the cervix continues to go through changes that allow it to better accommodate sperm (Thiyagarajan, 2020).

After ovulation, levels of estrogen begin to fall (Thiyagarajan, 2020).

The luteal or secretory phase

The next phase of the menstrual cycle takes place from day 14 through day 28. The dominant hormone in this phase is progesterone. Its purpose is to prepare your uterus for the possibility of a fertilized egg (Thiyagarajan, 2020). 

The endometrium prepares by increasing its blood supply and creating more mucus. It develops more complex glands, stores energy, and creates more surface area. Additionally, progesterone causes your body’s temperature to increase slightly (Thiyagarajan, 2020).

If you become pregnant, a fertilized egg will implant into your endometrium, and your hormone levels will stay elevated. However, if you don’t get pregnant that month, your levels of estrogen and progesterone drop, and your uterus will start to break down the endometrium. It will leave the body as your next period (Thiyagarajan, 2020). 

Normal symptoms of menstruation

Premenstrual syndrome (PMS) is a combination of symptoms that many women get about a week or two before their period. Studies have estimated that about 80% to 90% of women experience at least one symptom of PMS (Gudipally, 2021).

Common symptoms of PMS include (Gudipally, 2021):

  • Changes in appetite

  • Weight gain or bloating

  • Abdominal pain or cramping

  • Low back pain

  • Breast swelling and tenderness

  • Nausea

  • Constipation

  • Anxiety

  • Irritability or anger

  • Fatigue

  • Restlessness

  • Mood swings

About 2.5% to 3% of women experience symptoms of PMS that are severe enough to affect their daily activities (Gudipally, 2021). 

Another relatively common symptom of menstruation is menstrual-related headaches (MRH). These are a class of headaches that can occur in women due to a decline in estrogen that happens during the menstrual cycle. If they occur, they usually start two days before the start of your menstrual period and continue through the third day of menstrual bleeding (Moy, 2021).

Common remedies for period symptoms

There are many different methods for managing your menstrual period and its associated symptoms. You may need to try a few options before finding the one that works best for your menstrual flow and lifestyle. Choices include:

  • Disposable pads and liners

  • Reusable cloth pads and liners

  • Tampons

  • Menstrual cups

  • Menstrual discs

If you experience PMS symptoms that get in the way of your daily functioning, there are many different treatments available. You will likely benefit the most from a combination of treatment strategies (Gudipally, 2021).

Pharmacological options include (Gudipally, 2021):

Lifestyle modifications may also help you manage menstrual symptoms. These include (Gudipally, 2021):

  • Regular exercise

  • Massage therapy

  • Light therapy

  • Managing stress

  • Maintaining a healthy sleep routine

  • Increasing intake of complex carbohydrates

The fruit extract Vitex agnus-castus is the only herbal supplement that has been proven to help control menstruation-related mood swings and irritability (Gudipally, 2021).

When to see a healthcare provider

Most of the symptoms of normal menstruation can be managed by lifestyle changes or over-the-counter medications. However, if your menstrual symptoms interfere with your daily life, you should see your primary healthcare provider or a gynecologist.

One of the most common problems associated with the menstrual cycle is dysmenorrhea or period pain, which can cause people to miss days of work or school or miss out on other activities. The good news is you can usually treat it with medications (Nagy, 2021).

Abnormal uterine bleeding (AUB) is a broad term that includes any irregularity in your period's frequency, duration, and volume (not including during pregnancy). Up to one third of all women will experience AUB at some point, most commonly around the time of your first period or near menopause. A healthcare provider should assess you to figure out what is causing AUB. The treatment will depend on the underlying cause (Davis, 2021).

Additional reasons to see your healthcare provider include if your period lasts longer than ten days or you experience fewer than four periods per year. Several health conditions can cause previously regular periods to become irregular. Your provider can help you get a diagnosis and treat them (Riaz, 2021).


If you have any medical questions or concerns, please talk to your healthcare provider. The articles on Health Guide are underpinned by peer-reviewed research and information drawn from medical societies and governmental agencies. However, they are not a substitute for professional medical advice, diagnosis, or treatment.

How we reviewed this article

Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

Current version

September 03, 2021

Written by

Ellyn Vohnoutka, BSN, RN

Fact checked by

Steve Silvestro, MD

About the medical reviewer

Dr. Steve Silvestro is a board-certified pediatrician and Associate Director, Clinical Content & Education at Ro.