Menstrual Cycle & Period Calculator
Calculate your next period, fertile window, and hormonal phase timing based on your average cycle length and last period date.
Menstrual Cycle Guide
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 pregnan
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 (menorrhagia): worth investigating with GP —
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): severe form affecting approximately 3-8%. Significantly disruptive mood symptoms including depression, anxiety, irritability, and emotion
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% typic
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