The menstrual cycle is usually described by its total length, which obscures how it is built. It is two distinct phases separated by ovulation, and the two behave very differently.
The first half is driven by oestrogen
The follicular phase begins with menstruation. Follicle-stimulating hormone from the pituitary prompts a group of ovarian follicles to develop, and one becomes dominant.
That developing follicle produces oestrogen, and levels climb steadily through the phase.
Oestrogen thickens the uterine lining and changes cervical mucus, making it thinner and more permeable as the phase progresses.
Ovulation is triggered by a hormonal switch
Rising oestrogen normally suppresses the pituitary. Once it stays high enough for long enough, the relationship inverts and the pituitary releases a sharp surge of luteinising hormone.
That surge causes the dominant follicle to rupture and release its egg, generally within a day or so of the peak.
Home ovulation tests detect this surge, which is why they indicate an event about to happen rather than one already completed.
The second half is driven by progesterone
The emptied follicle becomes the corpus luteum, a temporary structure that produces progesterone.
Progesterone changes the uterine lining from growing to secretory, preparing it to support implantation, and raises resting body temperature slightly.
If implantation does not occur, the corpus luteum breaks down on its own schedule, hormone levels fall, and the lining is shed.
Variability lives almost entirely in the first phase
The luteal phase is relatively fixed in length across cycles and across individuals, because it is governed by the lifespan of the corpus luteum.
The follicular phase is not fixed. How long a follicle takes to mature varies with age, illness, significant stress and other factors.
A cycle that runs long or short is therefore usually a follicular phase that ran long or short, which is why calendar-based timing of ovulation is unreliable.
What counts as irregular is a clinical question
Some cycle-to-cycle variation is ordinary, and cycles are frequently anovulatory in the years after menarche and again as menopause approaches. A cycle without ovulation has no luteal phase to fix its length, so it can run unusually long.
Temperature charting and ovulation tests each capture only one part of the sequence, which is why neither on its own confirms that a cycle was ovulatory.
Cycles that are consistently very long or very short, bleeding between periods, or a sudden change in an established pattern are worth investigating with a doctor rather than tracking indefinitely.