When is your next period? When are your fertile days? Your menstrual cycle affects daily life in many ways — and understanding it helps you plan ahead.
This article explains how the cycle works, what phases it goes through, and how a period calculator can predict your next periods.
The 4 Phases of the Menstrual Cycle
The menstrual cycle has four distinct phases, each driven by different hormones and accompanied by specific physical changes.
1. Menstrual Phase (Days 1–5)
The uterine lining sheds — this is your period. Day 1 is the first day of bleeding. Duration varies between 3 and 7 days. Progesterone and estrogen are at their lowest.
2. Follicular Phase (Days 6–13)
Follicle-stimulating hormone (FSH) triggers follicle growth in the ovaries. Estrogen rises, rebuilding the uterine lining. This phase is variable — shorter cycles have a shorter follicular phase, longer cycles a longer one.
3. Ovulation (around Day 14)
A surge in luteinizing hormone (LH) triggers ovulation. The mature egg is released and travels into the fallopian tube. The fertile window spans the 5 days before ovulation and ovulation day itself.
4. Luteal Phase (Days 15–28)
The empty follicle becomes the corpus luteum and produces progesterone. If the egg isn't fertilized, progesterone drops after about 14 days — and your period begins. This phase is relatively constant for most women.
How to Predict Your Next Period
Prediction uses the calendar method. You need the first day of your last period and your average cycle length.
Formulas
Next period = Last period start + Cycle length
Ovulation = Next period − 14 days
Fertile window = Ovulation −5 days to ovulation day
Example: 28-Day Cycle
Last period March 1 → Next period March 29. Ovulation March 15 (day 14). Fertile March 10–15.
Example: 32-Day Cycle
Last period March 1 → Next period April 2. Ovulation March 19 (day 18). The follicular phase is 4 days longer; the luteal phase stays at 14 days.
Cycle Length: What's Normal?
A normal cycle lasts between 21 and 35 days. The average is 28 days, but only about 13% of women have an exact 28-day cycle.
Variations of 1–2 days between cycles are normal. To determine your average cycle length, track your cycle for at least 3–6 months.
Cycle Irregularities — When to See a Doctor
Occasional variations are normal. See a doctor if you experience:
Missed Periods (Amenorrhea)
When your period is absent for 3 months or longer (without pregnancy). Possible causes: stress, underweight, hormonal imbalances, PCOS.
Very Heavy or Prolonged Bleeding
Bleeding lasting over 7 days or needing to change pads/tampons every hour. May indicate fibroids, polyps, or clotting disorders.
Very Short or Very Long Cycles
Consistently under 21 days or over 35 days. May point to thyroid issues, PCOS, or ovarian dysfunction.
Severe Pain
When menstrual cramps disrupt daily life and painkillers aren't enough. A possible cause: endometriosis.
Benefits of Cycle Tracking
Knowing your cycle brings tangible benefits — whether or not you're trying to conceive.
Better Planning
Vacations, sports, important events — knowing when your period is due helps you organize daily life.
Early Detection of Changes
Regular tracking makes deviations visible — before they become a problem. Doctors appreciate documented cycle data.
Fertility & Contraception
Knowing your fertile days helps with natural family planning — whether you're trying to conceive or avoid pregnancy.
Understanding PMS & Mood Swings
When you know which cycle phase you're in, you can better contextualize mood swings, energy dips, and cravings.
Calculate Your Cycle Now
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Calculate for free →Conclusion
Understanding your cycle is the first step toward greater body awareness. The calendar method provides a good estimate for your next period and fertile days — but it's not a substitute for medical diagnosis.
If you experience persistent irregularities, severe pain, or have been trying to conceive for more than 12 months, consult your gynecologist.
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