The ovarian cycle is mostly controlled by the hormones oestrogen, progesterone, follicle-stimulating hormone (FSH) and leutinising hormone (LH). During a woman’s reproductive life, her ovaries release eggs (ova) in cycles. The release of an egg ready for fertilisation is known as ovulation. Each cycle lasts 28 days. Ovulation occurs around the 14th day of the cycle. Most women are fertile for 2-7 days of the month around the time of ovulation. You can identify these days by getting used to the changes in your cervical mucus or discharge. Many women experience a surge in their libido around the time of ovulation.
day 1-13: follicular phase
The ovarian cycle begins with the follicular phase. Low levels of oestrogen and progesterone cause the uterine lining to shed, this is known as menstruation. This marks the first day of the menstrual cycle.
During the first half of this phase, the follicle stimulating hormone (FSH) level increases stimulating the development of several follicles, each containing an egg. Only one follicle continues to develop. The egg is developing and maturing in the ovary on about the fifth day of the cycle. During the last part of this phase, the level of oestrogen secreted by the ovaries increases, which stimulates the uterine lining to thicken and rebuilds the endometrium.
day 10-18: ovulatory phase (fertile period)
A surge in luteinising hormone and follicle-stimulating hormone levels begins the ovulatory phase. Ovulation (release of the egg) usually occurs 16 -32 hours after the surge begins. After ovulation, under the influence of oestrogen and progesterone, the endometrium becomes thicker and spongy to receive a fertilised egg.
day 15-28: luteal phase
During the luteal phase, levels of luteinising hormone and follicle-stimulating hormone decrease. The ruptured follicle closes after releasing the egg and forms a corpus luteum, which secretes progesterone. Progesterone and oestrogen cause the endometrium to thicken. If the egg is not fertilised, the corpus luteum dies and no longer secretes progesterone. Because oestrogen and progesterone levels fall the endometrium is shed (menstruation). This begins a new menstrual cycle.