The menstrual cycle phases
A typical 28-day cycle has 4 phases. The menstrual phase (days 1-5) is bleeding. The follicular phase (days 1-13) overlaps with menstruation and continues as the uterine lining rebuilds and a follicle matures. Ovulation (days 13-15) is the release of the egg. The luteal phase (days 15-28) follows ovulation; if fertilisation doesn't occur, progesterone drops and menstruation begins.
Only the luteal phase is relatively fixed at 12-14 days. The follicular phase is what varies with cycle length. A 35-day cycle has a longer follicular phase, not a longer luteal phase.
What counts as irregular?
Cycles between 21-35 days are considered normal. Variation of up to 7-9 days between cycles is also within normal range for many women. Cycles consistently shorter than 21 days (polymenorrhoea) or longer than 35 days (oligomenorrhoea), or absence of periods for 90+ days (amenorrhoea), warrant clinical evaluation.
Related tools
For ovulation timing and fertile window, see Ovulation Calculator. If you've confirmed pregnancy, calculate your due date with the Due Date Calculator. For gestational weight gain targets, see Gestational Weight Gain. For BMI, see BMI Calculator.