A normal menstrual cycle is 21–35 days, measured from the first day of one period to the first day of the next. The commonly cited average is 28 days, but studies of large populations show the most common cycle length is actually 29 days. Cycles consistently outside the 21–35-day range may warrant evaluation for hormonal imbalance.

The menstrual cycle has four phases: (1) Menstruation (days 1–5 avg): shedding of the uterine lining. (2) Follicular phase (days 1–13): FSH stimulates follicle development; estrogen rises. (3) Ovulation (day 14 avg in a 28-day cycle): LH surge triggers egg release. (4) Luteal phase (days 15–28): corpus luteum secretes progesterone; if no implantation, progesterone drops and menstruation begins.

Normal period duration is 3–7 days. Average blood loss is 30–80 mL per cycle (one soaked regular pad or tampon = ~5 mL). Periods shorter than 3 days (hypomenorrhea) or longer than 7 days, or with blood loss exceeding 80 mL (menorrhagia), may indicate hormonal issues, fibroids, or a bleeding disorder.

Common causes of irregular periods include: stress (elevates cortisol, disrupting the HPO axis), significant weight loss or gain, intense exercise (female athlete triad), polycystic ovary syndrome (PCOS), thyroid disorders (hypo- or hyperthyroidism), perimenopause, hormonal contraceptives, and breastfeeding. A single irregular cycle is usually not concerning.

Premenstrual syndrome (PMS) affects up to 75% of women, causing physical symptoms (bloating, breast tenderness, headaches) and mood changes in the luteal phase (days 15–28). Premenstrual dysphoric disorder (PMDD) is a severe form affecting 3–8% of women, characterized by significant depression, anxiety, or irritability that impairs daily function. PMDD meets DSM-5 diagnostic criteria and responds to SSRIs or hormonal treatment.

The follicular phase runs from the first day of menstruation to ovulation (days 1–13 in a 28-day cycle). Follicle-stimulating hormone (FSH) recruits multiple follicles, but typically one dominant follicle matures. Rising estrogen causes the endometrium to thicken. The follicular phase varies in length (8–21 days), making it the primary driver of cycle-to-cycle variability.

The luteal phase runs from ovulation to the onset of menstruation. The ruptured follicle becomes the corpus luteum, producing progesterone (peaks ~7 days after ovulation). If no implantation occurs, the corpus luteum degrades, progesterone falls, and menstruation begins. Normal luteal phase: 12–16 days. A luteal phase under 10 days (luteal phase defect) may impact fertility.

Polycystic ovary syndrome (PCOS) affects 8–13% of reproductive-age women and is a leading cause of irregular periods. PCOS disrupts ovulation (anovulatory cycles), causing infrequent periods (oligomenorrhea, < 9 cycles/year) or absent periods (amenorrhea). It is associated with elevated androgens, insulin resistance, and multiple small ovarian follicles. Diagnosis uses Rotterdam criteria (2 of 3: irregular cycles, hyperandrogenism, polycystic ovaries on ultrasound).

Moderate exercise improves cycle regularity and reduces PMS symptoms. High-volume or high-intensity training can suppress the hypothalamic-pituitary-ovarian (HPO) axis through the female athlete triad (low energy availability, low bone density, menstrual dysfunction). Training over 10 hours per week without adequate caloric intake can delay or suppress periods. Recovery requires restoring energy balance.

Bright red blood indicates active, fresh flow. Dark brown or black blood is old blood that took longer to exit the uterus — common at the start or end of the period. Pink blood can indicate light flow or low estrogen. Gray or green-tinted discharge with unpleasant odor may indicate infection (bacterial vaginosis) and warrants medical evaluation. Occasional color variation is normal.

See a doctor if: periods are consistently shorter than 21 days or longer than 35 days; soaking a pad/tampon every hour for multiple consecutive hours; period is absent for 3+ months (not due to pregnancy); severe pain (dysmenorrhea) interfering with daily life; or you notice sudden significant changes in cycle length, flow, or symptoms. These may indicate PCOS, fibroids, endometriosis, thyroid disorders, or other conditions.

The standard calendar-method formula is: next period start date = last period start date + average cycle length (in days). This calculator applies exactly that formula using the last period date and cycle length you enter, then estimates ovulation as 14 days before that predicted next period (based on the typical ~14-day luteal phase). Because cycle length can vary from month to month, the prediction is an estimate, and it becomes less reliable the more irregular your cycles are.

A period tracker predicts your next period — and often your fertile window — from your cycle history, typically your last period start date and average cycle length. This calculator works the same way: enter your last period date and cycle length, and it projects your next period and estimated ovulation date using the calendar-method formula above. It calculates from the values you enter each time rather than storing a running log of past cycles.