Menstrual cycle phases and prediction science
A normal menstrual cycle is divided into four key physiological phases: the menstrual phase (bleeding, days 1 to 5), follicular phase (egg maturation, days 1 to 13), ovulation (egg release, day 14 in a 28-day cycle), and luteal phase (post-ovulation preparation, days 15 to 28).
The follicular phase exhibits significant variability between individuals and cycles, while the luteal phase remains relatively constant (typically 12 to 14 days).
Cycles between 21 and 35 days in adult women are classified as clinically normal. Cycle tracking enables early detection of hormonal shifts, pregnancy, or gynecological conditions.
Worked example: 3-month forecast
With a period start on October 1st and a consistent 29-day cycle:
Next cycle commences on October 30th, followed by November 28th and December 27th.
Ovulation is anticipated on days 15 of each cycle.
Period tracking inputs
Last Period Start Date
First day of menstrual bleeding.
Cycle Length
Duration in days between period start dates.
CALCULATOR QUESTIONS
Menstrual cycle & period predictor questions
Why does my period cycle length change from month to month?
Minor fluctuations of 2 to 4 days are completely normal and can be triggered by stress, travel, illness, sleep alterations, diet changes, or intense physical exercise.
When is a missed period considered late?
A period is typically considered late once 5 or more days have elapsed past your expected start date. If 60 or more days pass without bleeding, it is classified as amenorrhea.