The monthly menstrual cycle that leads to the period is a reality most women on Earth will go through in their lives.

But why is this cycle so universal? And what makes it a cycle in the first place?

Periods last anywhere between two and seven days, arising once within in a 28-day rotation.

That whole system occurs on repeat, happening approximately 450 times during a woman's life.

Behind the scenes are a series of hormonal controls that fine-tune the body's internal workings to make menstruation start or stop during those 28 days.

This inner machinery includes two ovaries stocked with thousands of tiny sacks called follicles that each contain one oocyte, an unfertilized egg cell.

At puberty, ovaries hold over 400 thousand egg cells but release only one each month, which results in pregnancy or a period.

Here's how this cycle unfolds.

Each month beginning around puberty, the hormone-producing pituitary gland in the brain starts releasing two substances into the blood: follicle-stimulating hormone and luteinizing hormone.

When they reach the ovaries, they encourage the internal egg cells to grow and mature.

The follicles respond by pumping out estrogen.

The egg cells grow and estrogen levels peak, inhibiting the production of FSH, and telling the pituitary to pump out more LH.

That causes only the most mature egg cell from one of the ovaries to burst out of the follicle and through the ovary wall.

This is called ovulation, and it usually happens ten to sixteen days before the start of a period.

The tiny oocyte moves along the fallopian tube.

A pregnancy can only occur if the egg is fertilized by a sperm cell within the next 24 hours; otherwise, the egg's escapade ends, and the window for pregnancy closes for that month.

Meanwhile, the now empty follicle begins to release progesterone, another hormone that tells the womb's lining to plump up with blood and nutrients in preparation for a fertilized egg that may embed there and grow.

If it doesn't embed, a few days later, the body's progesterone and estrogen levels plummet, meaning the womb stops padding out and starts to degenerate, eventually falling away.

Blood and tissue leave the body, forming the period.

The womb can take up to a week to clear out its unused contents, after which, the cycle begins anew.

Soon afterwards, the ovaries begin to secrete estrogen again, and the womb lining thickens, getting ready to accommodate a fertilized egg or be shed.

Hormones continually control these activities by circulating in ideal amounts delivered at just the right time.

The cycle keeps on turning, transforming each day and each week into a milestone along its course towards pregnancy or a period.

Although this cycle appears to move by clockwork, there's room for variation; women and their bodies are unique, after all.

Menstrual cycles occur at different times in the month, ovulation comes at various points in the cycle, and some periods last longer than others.

Menstruation even begins and ends at different times in life for different women, too; in other words, variations between periods are normal.

Appreciating these differences and learning about this monthly process can empower women, giving them the tools to understand and take charge of their own bodies.

That way, they're able to factor this small cycle into a much larger cycle of life.