Question 1

What is a normal menstrual cycle length?

Accepted Answer

A normal menstrual cycle ranges from 21 to 45 days, with the average being 28 days. However, a study by Bull et al. (2019) analyzing over 600,000 cycles found that only 13% of women have exactly 28-day cycles. Cycles between 24 and 38 days are considered clinically normal by ACOG. Variations of up to 7-9 days between cycles are common and generally not a cause for concern.

Question 2

How does the calculator predict my next period?

Accepted Answer

The calculator uses the calendar method: it takes the first day of your last period and adds your average cycle length to predict the next period start date. Ovulation is estimated at cycle length minus 14 days, based on research by Wilcox et al. (NEJM 1995), since the luteal phase is consistently ~14 days. The fertile window spans 6 days (5 days before ovulation + ovulation day), and PMS onset is estimated at 10 days before the next period.

Question 3

Can I use this calculator with irregular cycles?

Accepted Answer

This calculator provides the most accurate results for regular cycles. If your cycles vary by more than 7-9 days, the predictions will be less reliable. For irregular cycles, consider tracking over several months and using the average. Conditions like PCOS, thyroid disorders, stress, or significant weight changes can cause cycle irregularity. Consult your doctor if your cycles are consistently irregular.

Question 4

What is PMS and when does it start?

Accepted Answer

Premenstrual syndrome (PMS) is a set of physical and emotional symptoms that occur in the days leading up to menstruation. According to ACOG Practice Bulletin No. 15, PMS affects up to 75% of menstruating women. Symptoms typically begin 7-10 days before your period and include mood changes, bloating, breast tenderness, fatigue, and food cravings. They usually resolve within 1-2 days after bleeding starts.

Question 5

When is my fertile window?

Accepted Answer

Your fertile window spans approximately 6 days each cycle: the 5 days before ovulation and ovulation day itself. Research by Dunson et al. (2002) shows that conception probability peaks on the 2 days before ovulation. Sperm can survive up to 5 days in the reproductive tract, while the egg remains viable for only 12-24 hours after release. This calculator estimates your fertile window using the calendar method.

Question 6

When should I see a doctor about my period?

Accepted Answer

See a doctor if: your cycles are shorter than 21 days or longer than 45 days, you bleed for more than 8 days, you experience very heavy bleeding (soaking through a pad/tampon every hour), you have severe menstrual cramps that interfere with daily activities, you miss 3 or more consecutive periods (without being pregnant), or you experience bleeding between periods. These may indicate conditions requiring medical evaluation (ACOG, Reed & Carr, Endotext 2018).

Period Calculator — Menstrual Cycle

Understanding Your Menstrual Cycle: A Complete Guide

How does this calculator work?

Understanding cycle phases

What is PMS?

Irregular cycles and when to see a doctor

Scientific Sources

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