The Four Phases
A menstrual cycle is counted from the first day of bleeding to the day before the next period begins. It runs through four phases driven by hormonal shifts:
| Phase | Typical days | What happens |
|---|---|---|
| Menstrual | 1–5 | The uterine lining is shed; hormones are at their lowest |
| Follicular | 1–13 | Follicles mature and oestrogen rises; overlaps the menstrual phase |
| Ovulation | 14 | An LH surge releases an egg, which survives 12–24 hours |
| Luteal | 15–28 | Progesterone rises then falls, triggering the next period |
The day numbers assume a 28-day cycle. On longer or shorter cycles, the follicular phase stretches or compresses while the luteal phase stays close to 14 days.
What Counts as Normal
Cycle lengths from 21 to 35 days are considered normal in adults, and 21 to 45 days in the first years after menarche. Bleeding usually lasts 2 to 7 days. Total blood loss averages 30–40 ml — two to three tablespoons — despite feeling like far more.
Worth discussing with a clinician: cycles consistently outside 21–35 days, variation of more than 7–9 days between cycles, bleeding longer than 7 days, soaking a pad or tampon hourly for several hours, bleeding between periods, or no period for three months.
Why Cycles Shift
Stress, illness, travel across time zones, significant weight change, intense training and poor sleep all affect the hypothalamic signals that drive the cycle. A single irregular cycle after a stressful month is common and not concerning. A persistent pattern is worth investigating — PCOS, thyroid disorders and endometriosis are all frequently missed.
Tracking Beyond Dates
Recording symptoms alongside dates makes the record far more useful: flow heaviness, cramping, mood, energy, sleep and skin changes. Patterns emerge over three to six cycles that a single month cannot show, and the record is genuinely useful in a clinical appointment.
Frequently Asked Questions
Can I get pregnant during my period?
It is uncommon but possible, particularly with short cycles. Sperm survive up to five days, so intercourse late in a period can overlap with an early ovulation.
Why is my period late?
Pregnancy, stress, illness, weight change, travel, new medication, hormonal contraception and approaching perimenopause are the common causes. A single late period is usually not significant.
Do periods sync between people who live together?
The evidence does not support it. The original 1971 study has not replicated, and later analyses attribute the perception to chance overlap between cycles of differing lengths.