The Menstrual Cycle
The menstrual cycle begins when a girl reaches the age of puberty. It is the reproductive cycle that produces eggs for fertilisation. During the menstrual cycle the uterus (endometrium) prepares itself for implantation of a fertilised egg, if this does not occur the uterus lining is shed from the body; this is known as menstruation or a "period".
On average the menstrual cycle lasts between 28-35 days. Day 1 of the cycle begins on is the first day of bleeding (bleeding can last for 3-7 days) and the cycle ends just before the next menstrual period.
The menstrual cycle is carefully regulated by several hormones: Luteinizing Hormone (LH), Follicle-stimulating Hormone (FSH), and the female sex hormones Oestrogen and Progesterone.
The cycle can be divided into three phases: follicular (before the egg is released), ovulatory (egg is released), and luteal (after release of the egg).
1. Follicular Phase
- This phase begins on the first day of bleeding.
- The key aspect of this phase is the development of follicles in the ovaries.
- At the start of the follicular phase, the lining of the uterus is thick with fluids and nutrients intended to nourish an embryo (fertilised egg). If no embryo is present, oestrogen and progesterone levels are low. This causes the uterus lining to shed and menstrual bleeding occurs.
- The pituitary gland (found in the brain) increases its’ production of follicle-stimulating hormone (FSH). This hormone stimulates the growth of several follicles (each contains an egg) to develop in the ovaries.
- The levels of FSH hormone decreases and the follicles begin to secrete oestrogen. The follicle that develops first (the dominant follicle) secretes the most amount of oestrogen, and this secretion subsequently suppresses the development of the other follicles.
- The follicular phase roughly lasts about 13 or 14 days. The phase ends when the level of luteinizing hormone (LH) surges dramatically.
2. Ovulatory Phase
- This phase begins with the luteinizing hormone surge (LH surge). The level of FSH increases to a lesser extent.
- LH stimulates enzymes in the dominant follicle and along with the increased pressure causes the follicle to rupture and release the egg (ovulation).
- The egg travels into the fallopian tube, ready for fertilisation. The egg can survive for 12 to 24 hours after ovulation.
- The LH surge can be used as a measurement to determine when a woman is fertile. Around 12 to 24 hours after the egg is released, the LH surge can be detected by measuring the level of this hormone in urine.
- The ovulatory phase usually lasts 16 to 32 hours and ends when the egg is released.
3. Luteal Phase
- This phase begins after ovulation.
- It lasts about 14 days and ends just before a menstrual period, unless of course fertilisation occurs.
- In this phase the egg travels along the fallopian tube by wave like motions caused by the finger-like projections in the walls of the fallopian tube.
- The remainder of the ruptured follicle in the ovary closes after releasing the egg and forms a structure called a corpus luteum. The corpus luteum secretes large quantities of progesterone and oestrogen and prepares the uterus for fertilisation. Progesterone causes the endometrium to thicken, filling with fluids and nutrients to nourish the potential embryo it also causes the mucus in the cervix to thicken, so that sperm and bacteria are less likely to enter the uterus.
- Progesterone also causes body temperature to increase slightly during the luteal phase and remain elevated until a menstrual period begins. This increase in temperature can be used to estimate whether ovulation has occurred.
- LH and FSH levels fall back to low and steady levels.
- Oestrogen levels fall a little after the LH/FSH surge, but rise due to continued secretion of oestrogen and progesterone by the corpus luteum.
- The increase in oestrogen and progesterone levels causes milk ducts in the breasts to dilate. In turn the breasts sometimes swell and become tender.
- If the egg is not fertilised, the corpus luteum shrinks and begins to degenerate after 14 days (the corpus luteum is designed to die after 14 days). The unfertilised egg also dies and passes out of the uterus with the menstrual bleeding. Oestrogen and progesterone levels fall, bleeding starts and the uterine lining is shed. A new menstrual cycle begins.
- If the egg is fertilised the cells around the developing embryo begin to produce a hormone called human chorionic gonadotropin (hCG). This hormone rescues the corpus luteum and allows it to continue secreting progesterone and oestrogen, until the growing foetus can produce its’ own hormones. Note: Pregnancy tests are based on detecting an increase in the human chorionic gonadotropin level.