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Luteinising hormone is a gonadotropin released by the anterior pituitary under hypothalamic control. It acts on the gonads: in testes it stimulates steroid production by Leydig cells, while in an ovarian cycle its surge helps trigger ovulation and the transition to a corpus luteum.^[ref]

The signal is dynamic. A sustained low concentration and a brief surge need not have the same physiological meaning, and a sparse sampling schedule can miss a peak. The same hormone also acts in different cellular contexts, illustrating that a hormone's identity alone does not determine one universal response. Receptor expression, developmental state and the other gonadal signals contribute to the outcome.

The menstrual cycle coordinates recurring ovarian and uterine changes through hormonal feedback. Cycle day one is conventionally the first day of menstrual bleeding. The ovarian sequence includes follicular development, ovulation and a luteal phase; the uterine lining undergoes shedding, proliferation and secretory changes in response to the changing signals.^[ref]

These are linked sequences, not identical names for one anatomical event. Follicular development concerns the ovary, while endometrial proliferation concerns the uterus. Luteinising hormone, ovarian estrogens and progesterone couple the two. A twenty-eight-day schematic is a teaching example rather than a rule that every cycle has that duration or ovulates on day fourteen. Comparing observations requires specifying the event or phase used to align the records.

Ovulation is release of an oocyte from an ovarian follicle in the ovary. In the human menstrual cycle, rising luteinising hormone precedes this event; the released oocyte can subsequently encounter sperm in a fallopian tube. Ovulation and fertilization are separate events: releasing an oocyte does not establish a pregnancy.^[ref]

Calendar day alone is an imperfect marker because the timing of ovulation varies among individuals and cycles. An observed hormonal change measures part of the regulatory sequence rather than directly observing fusion of gametes.