What your cycle is actually doing

Your cycle length is counted from the first day of one period to the day before the next one begins - not from the day a period ends. It is the single most common place the counting goes wrong, and it makes cycles look shorter than they are.

In adults a cycle is often somewhere between 21 and 35 days. In the first few years after periods begin it is commonly longer and far less predictable, sometimes up to about 45 days, and it usually settles with time. Cycles change again in the years approaching menopause.

They are described as four, but they overlap and only two of them are events with a clear edge.

• Menstruation. Hormone levels have fallen and the lining built up over the last cycle leaves the body. Commonly two to seven days.
• Follicular phase. Begins on day 1, so it runs underneath your period. Follicles in the ovary mature, usually with one becoming dominant, estrogen rises and the lining rebuilds.
• Ovulation. A surge in luteinising hormone releases the mature egg.
• Luteal phase. The emptied follicle becomes the corpus luteum and releases progesterone, holding the lining in place until hormone levels drop and the next period starts.

Day 14 comes from a textbook 28-day cycle, and it has misled a great many people. Ovulation is anchored to the period that follows it, not the one before: it usually happens around 12 to 16 days before your next period starts.

So in a 35-day cycle, ovulation is nearer day 21 than day 14. The luteal phase - from ovulation to the next period - is the steadier half. The follicular phase is what stretches and shrinks, and it is the main reason two cycles are rarely the same length.

Not every cycle includes ovulation, and an occasional cycle without it is not unusual on its own.

Writing down the first day of each period for a few months tells you more than any average. You learn your own range, and a change from your own range is far more informative than a comparison to someone else.

It also makes an appointment more useful. "My cycles were 28 to 30 days and for the last four months they have been 45" is a sentence a clinician can work with. "They feel irregular" is much harder.

Cycle apps and calculators estimate. They cannot see whether you ovulated, and they should not be relied on as contraception.