The Menstrual Cycle: Quick Guide

Menstruation is the elimination of the thickened lining of the uterus (endometrium) from the body through the vagina. This flow is usually a sign a pregnancy was not achieved. Menstrual fluid contains blood, cells from the lining of the uterus (endometrial cells) and mucus. The average length of a period is between three and seven days.

The follicular phase starts on the first day of menstruation and ends with ovulation. During the follicular phase, a follicle on the ovary gets ready to release an egg. The following events occur during this phase:

• Prompted by the hypothalamus, the pituitary gland releases follicle stimulating hormone (FSH).
• During the first few days of the cycle, through the influence of a rise in follicle stimulating hormone (FSH), a number of follicles are stimulated.
• Only one dominant follicle containing the egg will mature (sometimes two).
• The growth of these follicles stimulates the lining of the uterus to thicken in preparation for a possible pregnancy.

Ovulation is the release of a mature egg from the surface of the ovary. This generally occurs mid-cycle, around two weeks before menstruation starts. During this phase the following events occur:

• During the follicular phase, the developing follicle causes a rise in the level of estrogen.
• The hypothalamus in the brain recognises rising levels of estrogen and releases a chemical called gonadotrophin-releasing hormone (GnRH).
• The gonadotrophin-releasing hormone (GnRH) prompts the pituitary gland to produce raised levels of luteinising hormone (LH) and follicle stimulating hormone (FSH).
• A rapid surge of luteinising hormone (LH) is released by the anterior pituitary gland, which triggers ovulation and causes the dominant mature follicle to release an egg.

After being released from the ovary, the egg is swept up into the far end of the fallopian tube. If fertilisation does not take place within 12-24 hours of ovulation, the egg starts to disintegrate. If sperm are present in the fallopian tube, the egg may be fertilised and begin the process of embryo development. The developing embryo may take three to five days to reach the uterus and implant into the endometrium, which may result in a pregnancy.

During ovulation, the egg bursts from its follicle. The ruptured follicle stays on the surface of the ovary and becomes the corpus luteum. The corpus luteum produces the hormones progesterone and estrogen to provide nutrition to the endometrium (lining), preparing the uterus for a fertilised egg to implant.

If a fertilised egg implants in the lining of the uterus, it starts to produce hormones necessary to maintain the corpus luteum. This includes human chorionic gonadotrophin (HCG), which is detected in a urine pregnancy test. The presence of this hormone causes the ovary to continue producing progesterone to support the pregnancy.

If a pregnancy does not occur, the corpus luteum will perish around day 22 in a 28-day cycle. The drop in progesterone levels causes the lining of the uterus to shed and the next period begins.