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Understanding Your Menstrual Cycle: Why Your Period Arrives When It Does

Understanding Your Menstrual Cycle: Why Your Period Arrives When It Does at The Kingston Pharmacy, Kingston upon Thames

A simple guide to the menstrual cycle, the hormones that control your periods, and how period delay treatment works within the natural cycle.

What Is the Menstrual Cycle?

The menstrual cycle is the series of hormonal changes that occur in the body each month to prepare for a possible pregnancy. Although many people think of a period as the menstrual cycle itself, a period is actually just one part of the overall cycle.

During each cycle, hormones work together to prepare an egg for release from the ovary, build up the lining of the womb, and support a possible pregnancy. If pregnancy does not occur, hormone levels change and the lining of the womb is shed — this is what causes a period. The cycle then begins again.

How Long Is a Normal Menstrual Cycle?

A normal menstrual cycle can vary from person to person. For most women, cycles occur every 21 to 35 days, with the average lasting around 28 days. Small variations from month to month are often completely normal.

  • Stress
  • Illness
  • Changes in weight
  • Travel
  • Exercise levels
  • Hormonal changes

All of these factors can affect when a period arrives. It is important to remember that not everyone’s cycle follows a perfectly predictable pattern.

The Four Phases of the Menstrual Cycle

Understanding the different stages of the cycle can help explain why periods happen and how treatments such as norethisterone work.

Phase 1
Menstrual Phase

The stage when you have your period. The womb lining built up during the previous cycle is shed and leaves the body through vaginal bleeding. For most women, periods last between 3 and 8 days.

Phase 2
Follicular Phase

While your period is ending, your body begins preparing for the next cycle. Hormones stimulate the ovaries to develop follicles containing immature eggs, and the womb lining starts to thicken again in preparation for a possible pregnancy.

Phase 3
Ovulation

Usually occurs around the middle of the cycle. A mature egg is released from the ovary — this is generally the most fertile stage of the menstrual cycle. If the egg is fertilised, pregnancy can occur.

Phase 4
Luteal Phase

After ovulation, the body produces higher levels of progesterone to maintain the womb lining and prepare for a possible pregnancy. If pregnancy does not occur, progesterone levels begin to fall — triggering the next period.

What Hormones Control Your Cycle?

Several hormones work together throughout the menstrual cycle to create the regular pattern that most women experience each month.

Oestrogen

Helps develop and thicken the womb lining after a period, and plays an important role in preparing the body for ovulation.

Progesterone

The dominant hormone after ovulation. Maintains the womb lining and supports a possible pregnancy. When levels fall, the body receives the signal to start a period.

FSH

Follicle-stimulating hormone. Helps stimulate the growth of follicles within the ovaries, which contain the eggs that may later be released during ovulation.

LH

Luteinising hormone. Triggers ovulation and the release of a mature egg from the ovary.

Why Does a Period Happen?

A period occurs because hormone levels change. After ovulation, progesterone levels remain high for a short time. If pregnancy does not occur, progesterone levels begin to fall. The body responds by shedding the lining of the womb, resulting in menstrual bleeding.


This process repeats each month as part of the normal reproductive cycle.

Why Do Periods Sometimes Arrive Early or Late?

Many women experience occasional changes in cycle timing. While occasional variations are common, persistent changes should be discussed with a healthcare professional.

Stress  — can affect the hormones that regulate ovulation and menstruation
Travel  — changes in routine, sleep patterns, and time zones can influence cycle timing
Weight changes  — significant weight gain or loss may affect hormone levels
Intense exercise  — very high levels of physical activity can sometimes alter menstrual cycles
Illness  — even short-term illness can occasionally delay or affect a period

How Period Delay Treatment Works Within the Menstrual Cycle

Period delay treatment works by influencing the hormonal changes that normally trigger a period. Norethisterone is a type of medicine called a progestogen, which acts in a similar way to the body’s natural hormone progesterone.

Normally, a period starts when progesterone levels begin to fall at the end of the cycle. Norethisterone maintains hormone levels and prevents this normal hormonal drop from occurring, meaning the body does not receive the signal to shed the womb lining — and the period is delayed. Once treatment is stopped, hormone levels fall and a period will usually begin within a few days.

This is why period delay treatment needs to be started before your period is due to begin — ideally three days before your expected period date.

What to Do Next

Understanding your menstrual cycle can help you make informed decisions about your reproductive health and better understand why your periods occur when they do.


If you have an upcoming holiday, special occasion, sporting event, or important commitment and would like to delay your period, period delay treatment may be an option. By completing our online assessment, one of our clinicians can review your medical history and determine whether treatment is suitable for you — ensuring you receive safe, personalised advice tailored to your individual needs.

Ready to get started with Period Delay Treatment?

Complete a short consultation and our team will take care of the rest.

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