Explained | Why do half the world population have periods?

World Menstrual Hygiene day 2026: A large part of menstrual hygiene is also to understand what causes the periods to occur, its biological timing and the nature of the blood flow.

Women across the globe have normal and abnormal menstrual bleeding – how to differentiate if one doesn’t know what normal should look like.

A large part of menstrual hygiene is also to understand what causes the periods to occur, its biological timing and the nature of the blood flow.

All this information can enable a person to spot any changes and seek medical advice if they notice something unusual.

While most women understand periods as a natural phenomenon, few understand the biological reality of it.

As per a UNICEF report, about 71 percent of Indian adolescent girls remained unaware about menstruation before they started with their first cycle.

Likewise, a study published in the National Library of Medicine showed that only 16 percent of participating adolescent girls knew that bleeding occurred from an organ called uterus.

Also, Youth ki Awaaz, a large online platform for young people to share stories and opinions, found that one third of their participants didn’t understand periods fully.

Hence, it can be safely said that period literacy remains low in the country.

For the longest, period blood has been placed equivalent to bodily waste like urine or feces across many cultures.

For years activists and experts have been fighting to make people believe that there is nothing pathological or contaminated about the period blood.

Why do people have periods?

The answer is simple — the uterus wants them to get pregnant and gets upset when they don’t.

As per Cleveland Clinic, one can understand menstruation or periods as the shedding of the uterus lining.

A uterus is a highly muscular, elastic organ that sits between the bladder and the rectum. This is where the fetus grows up during the pregnancy.

One can imagine the uterus as a warm host waiting patiently for a guest, increasing supplies of food and drinks, refilling the empty stocks to make the guest feel- like -home. When the guest fails to arrive, the uterus tends to throw away all the preparation in a fit of disappointment. Just as it does so, it starts to prepare for another guest with a renewed enthusiasm.

The guest is a potent embryo, formed by the fusion of a female egg and a male sperm and the shedding is the thick uterine lining developed to support the implantation of the embryo on the uterine wall and later support its progression into a fetus.

In many other animals, this developed tissue for the embryo is reabsorbed by the body — human bodies decided to push it out instead.

In all its preparations, the uterus is assisted by a cascade of hormones making these changes possible, like estrogen, progesterone, follicle stimulating hormone (FSH), prostaglandins, oxytocin and some others.

All these changes don’t happen in those five days alone - they extend over an average of 28 days, with this length varying for each menstruating person. 

For some it could be 24 weeks, for others it can be a bit longer, maybe 35 weeks.

The phases

The visible tissue shedding at the end is just a small part of the menstrual cycle with the entire process demarcated into four phases.

• Menses phase – This is the phase one recognises as ‘periods’ where the pregnancy doesn’t happen and hormones supporting the uterine tissue like estrogen and progesterone crash down. The uterus isn’t able to hold the rich, blood supplied tissue and throws it away through the vagina. It is assisted by prostaglandins and oxytocin which induce minor contractions in the uterus that push the tissue out. This phase can vary between two to five days on an average.

• Follicular phase - This phase overlaps with the menses phase where certain hormones promote the formation of protective sacs called follicles in the ovaries which hold the immature eggs. Only the healthiest egg gets matured and becomes ready to be released. The uterus also starts to get a fresh supply of blood and nutrients to support an anticipated pregnancy. This phase on an average lasts for 16 days, however, it varies for every menstruating person.

• Ovulation phase - In this short-lived phase, either of the two ovaries releases a matured egg into the fallopian tube, expecting a sperm to fertilize it. It happens somewhat near the middle of the menstrual cycle.

• Luteal phase - Some more hormones (estrogen and progesterone) are pumped into the bloodstream to make the uterine environment even richer in blood and nutrients.

If a person fails to get pregnant, the luteal phase progresses back into the menses phase and the cycle continues. If pregnancy occurs, the menstrual cycle gets interrupted and hits a temporary pause.

What not to expect?

The menstrual cycle is not the same for all females. It can vary in length, volume of blood shed and even days it can take to shed the lining.

However, most should be somewhere around the standard average.

There are few things one must always get checked with a doctor:

• When the menstrual cycle gets shorter than 21 weeks and more than 35 weeks.

• Not having a period for three months.

• Periods that are accompanied by heavy bleeding, pain and discomfort.

• Periods that last longer than seven days or require pad changing after every few hours.

• If bleeding happens between two consecutive menses phases.