Menstrual Cycle & Period Calculator

Menstrual Cycle Guide

What should I know about The Four Phases?

The menstrual cycle has four phases: Menstruation (days 1-5 typical): shedding of the uterine lining. Day 1 = first day of bleeding. Follicular phase (days 1-13): rising oestrogen. Follicles in the ovary develop. Energy, clarity, and motivation often peak in the late follicular phase. Ovulation (day 14 for a 28-day cycle): surge in LH triggers egg release. The egg survives 12-24 hours. Luteal phase (days 15-28): corpus luteum produces progesterone. Prepares uterus for implantation. If no pregnancy occurs, progesterone levels fall, the uterine lining sheds, and the next period begins, starting the cycle over again.

What should I know about Cycle Variations?

Average cycle length: 21-35 days (both extremes within normal range). Many conditions cause variation: short cycles (< 21 days): can indicate perimenopause, hormonal imbalance, stress. Long cycles (> 35 days): PCOS (polycystic ovary syndrome), hypothyroidism, high stress, low body weight. Variable cycle length: normal variation of ±2 days between cycles is common. More than ±7 days variation suggests investigating with a GP. Very heavy or long periods, known as menorrhagia, are worth investigating with a GP — they can sometimes indicate an underlying condition that's straightforward to treat once identified.

What should I know about PMS and PMDD?

PMS (premenstrual syndrome): physical and psychological symptoms in the luteal phase. Affects up to 75% of people who menstruate to some degree. Common symptoms: mood changes (irritability, low mood), bloating, breast tenderness, headaches, fatigue, food cravings. Typically resolve within 4 days of menstruation starting. PMDD (premenstrual dysphoric disorder, or PMDD): a severe form affecting approximately 3-8% of menstruating people, involving significantly disruptive mood symptoms including depression, anxiety, irritability, and emotional volatility in the days before a period.

What should I know about Tracking for Health Insights?

Benefits of menstrual cycle tracking: identifying cycle irregularities and trends. Understanding personal hormonal patterns. Recognising PMS/PMDD patterns. Fertility awareness (with appropriate education). Identifying health changes (irregular periods can indicate thyroid issues, PCOS, perimenopause). Popular tracking apps: Clue, Flo, Apple Health Cycle Tracking (built-in). Important: fertility awareness methods (FAM) using cycle tracking alone have high failure rates as contraception (24% typical-use failure rate per year — far higher than most other contraceptive methods).

What should I know about the Four Phases of the Menstrual Cycle?

The menstrual cycle is generally described in four overlapping phases driven by a changing balance of hormones. The menstrual phase (typically days 1-5) is when the uterine lining sheds as a period, triggered by a drop in progesterone and oestrogen from the previous cycle. The follicular phase overlaps with and continues after the period (roughly days 1-13 in an average cycle), during which rising follicle-stimulating hormone (FSH) prompts several ovarian follicles to develop, one of which becomes dominant, while oestrogen rises and begins rebuilding the uterine lining. Ovulation (typically around day 14 in a 28-day cycle, but varying by individual) is triggered by a surge in luteinising hormone (LH) and is the brief window when the dominant follicle releases its egg. The luteal phase (roughly days 15-28) follows, during which the ruptured follicle transforms into the corpus luteum, producing progesterone to maintain the uterine lining in case of pregnancy; if no pregnancy occurs, the corpus luteum breaks down, progesterone and oestrogen fall sharply, and the cycle begins again with a new period. This hormonal rhythm is also what drives many of the physical and mood changes commonly associated with different points in the cycle.

What do I need to know about When Irregular Cycles Need Medical Review?

Day-to-day variation in cycle length of a few days is entirely normal, but certain patterns of irregularity are worth discussing with a GP rather than just continuing to track. These include cycles that are consistently shorter than 21 days or longer than 35 days, periods that stop for three months or more without pregnancy, breastfeeding, or menopause as an explanation, bleeding between periods or after sex, and periods that suddenly become significantly heavier, lighter, or more painful than someone's usual pattern. Such changes can have a range of underlying causes, including polycystic ovary syndrome (PCOS, which affects an estimated 1 in 10 women of reproductive age in the UK and commonly causes infrequent or irregular ovulation), thyroid dysfunction, significant weight changes, high stress, perimenopause, uterine fibroids or polyps, or certain medications and hormonal contraceptives. A cycle tracker is genuinely useful here: a few months of logged dates, flow and symptoms gives a GP much more to work with than a vague recollection of 'my periods have been weird lately', and can help distinguish a one-off irregular cycle (common and usually not concerning) from an established pattern that warrants investigation. Tracking apps are a record-keeping tool, not a diagnostic one, so persistent irregularity is still best raised directly with a healthcare professional.

What should I know about Tracking Apps vs Natural Family Planning Reliability?

Period and fertility tracking apps estimate ovulation using calendar-based calculations from past cycle lengths, which is a reasonable way to understand general cycle patterns but is considerably less precise than formal, clinically-validated Natural Family Planning (NFP, sometimes called fertility awareness methods) when used specifically for contraception or conception planning. Properly taught NFP methods combine multiple physical signs — basal body temperature, cervical mucus changes, and sometimes cycle length calculations together — cross-checked against each other daily, which is why, when taught by a trained practitioner and followed carefully, these methods have documented effectiveness rates. A calendar-based app prediction alone, especially for someone with naturally variable cycle lengths, can be off by several days in either direction, since it's typically predicting based on averages from previous cycles rather than detecting the actual current cycle's signs in real time. This distinction matters most for anyone relying on cycle tracking specifically to avoid or achieve pregnancy rather than for general health awareness: for contraceptive reliability, properly taught fertility awareness methods combining multiple signs are a meaningfully different, more rigorous approach than a calendar app's single-signal prediction, and anyone relying on tracking for contraception should seek guidance on a validated method rather than an app alone.

Worked Example: Projecting the Next Three Cycle Dates

For someone with a fairly regular cycle, projecting future cycle dates is a matter of repeatedly adding the average cycle length (counted from the first day of one period to the first day of the next) to the most recent period start date — though this projection becomes less reliable the further ahead it is used, since even typically-regular cycles vary by a few days from one month to the next.

Worked example: if a period started on 3 October and the average cycle length over the past several months has been 28 days, the next period would be projected for 31 October (3 October + 28 days), the one after that for 28 November, and the one after that for 26 December. If the actual average cycle length were 30 days instead of 28, each of these projected dates would shift roughly two, four, and six days later respectively — which is why tracking several consecutive cycles (rather than assuming a textbook 28-day cycle) gives a noticeably more accurate personal projection than a single generic calculation.

How Much Does Stress Genuinely Delay a Period?

Significant physical or psychological stress can delay ovulation by disrupting the hypothalamic-pituitary-ovarian axis that regulates the menstrual cycle, which in turn delays the period that follows, since a period is triggered by the hormonal drop that follows ovulation rather than occurring on a fixed calendar schedule regardless of what happens earlier in the cycle. This is a genuine, well-documented physiological pathway rather than just a commonly repeated claim — major life stressors, significant weight change, intensive exercise training, illness, and travel-related disruption to sleep patterns can all push ovulation (and therefore the next period) later than a typical cycle length would predict. A single delayed cycle from an identifiable stressful period is generally not a cause for concern, but cycles that are consistently irregular for reasons unrelated to any obvious one-off stressor are better assessed with a GP, as covered above.