Regularity of menses

Regular and irregular frequency of menses. "Regular" means "cyclically predictable" and generally reflects the presence of consistently predictable ovulation, typically about 14 days prior to the onset of menses.
(A) The top panel depicts a 65-day interval where the first cycle length is 29 days and the second is 31 days, with each menstrual period lasting 5 days and day 2 having the heaviest volume. This variation of 2 days is well within the definition of regularity: cycle length from first day of one period to the next is fairly consistent, with each cycle length within 4 days of the others.
(B) The bottom panel demonstrates an irregular cycle, with the first cycle a length of 13 days and a second cycle of 39 days. This variation in cycle length is outside the normal range for any age and suggests that the patient is not ovulating regularly or not ovulating at all. Note the variable duration and flow of the menses, also consistent with some ovulatory disorders, but not necessary for defining "irregular" onset of menses.