The menstrual cycle involves a series of hormonal events which occur at fairly regular intervals. The average menstrual cycle is approximately 28 days, although this may vary considerably between women. The menstrual cycle involves four distinct phases:
Day 1-5: menstruation (the menstrual period);
Day 3-13: the proliferative or follicular phase;
Day 14: ovulation;
Day 15-28: the luteal or secretory phase.
Although the first day of menstruation is usually referred to as the start of the menstrual cycle, the menstrual period (days 1-5) is actually the culmination of the hormonal changes which make up the menstrual cycle.
Luteal or secretory phase-The luteal phase extends from ovulation to menstruation. It is the phase during which a variety of substances are secreted by the endometrium in order to prepare it for implantation of the fertilised ovum. It is sometimes also known as the secretory phase.
Immediately after ovulation occurs the remains of the ruptured follicle are transformed into a structure known as the corpus luteum. The corpus luteum then begins to produce progesterone and a small amount of oestrogen.
The progesterone causes the endometrium to thicken even more. It also stimulates the cells in the endometrium to secrete substances that will nourish the fertilised ovum if pregnancy occurs.
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