The average length of the menstrual cycle is 28 days, but this can vary from one woman to the next and between each cycle. The length of a menstrual cycle is calculated from the first day of your period to the day before the next period starts.

The menstrual cycle is more than just a period and is actually made up of two cycles controlled by hormones - one that happens in the ovaries and one that happens in the uterus.

Hormones during the menstrual cycle 

The menstrual cycle is a complex system in which different glands produce different hormones to prompt various processes in the body which are all interlinked. The hypothalamus in the brain sends a message to the pituitary gland to produce certain chemicals, which then prompt the ovaries to produce oestrogen and progesterone.

There are two main parts to the menstrual cycle. The first part of the cycle prepares an egg to be released from the ovaries and builds the uterine lining. The second part of the cycle prepare the uterus for implanting a fertilised egg, or if pregnancy doesn’t happen, to shed the uterine lining by menstruation and start the next cycle.

Phases of the menstrual cycle

The main phases of the menstrual cycle are:

• menstruation
• follicular phase (in the ovaries)
• proliferative phase (in the uterus)
• ovulation
• luteal phase (in the ovaries)
• secretory phase (in the uterus)

The follicular and proliferative phases, occur at similar times but one occurs in the ovaries and the other in the uterus. The same is true for the luteal phase and secretory phase.

Menstruation

Menstruation is the shedding of the thickened uterine lining from the body through the vagina. This fluid contains blood, cells from the uterus lining (endometrium) and mucus. The average length of a period is between three days and one week. Levels of oestrogen and progesterone hormones at this stage are low. 

Follicular phase

The follicular phase takes place between the first day of menstruation and the day of ovulation. During this phase the pituitary gland releases follicle stimulating hormone (FSH) which stimulates the ovaries to produce a number of follicles.

The follicles are fluid filled sacs that each contains an immature egg. Halfway during this phase, one of the follicles will be larger than the others and will become the dominant follicle. This follicle produces oestrogen as it grows and will be the one to release the mature egg at ovulation, while the others will die.

The level of estrogen peaks just before ovulation, and in turn causes the peak of the luteinising hormone (LH). This LH surge causes the release of the mature egg from the ovary and is what ovulation predictor kits try to pick up.

Proliferative phase

While the ovaries are going through the follicular phase, the uterus is in the proliferative phase building back the uterine lining.

The uterus responds to the estrogen produced by the follicles and rebuilds the lining in preparation for accepting a fertilised egg. The uterine lining (endometrium) is the thinnest during/just after menstruation and gradually thickens until the thickest point at ovulation.

Ovulation

Ovulation is the release of a mature egg from a mature follicle in the ovaries. This usually happens during the middle of the cycle, approximately two weeks before the start of the next menstruation.

The level of estrogen peaks and causes the brain to create a chemical called gonadotrophin releasing hormone (GnRH), which then causes a surge in the LH hormone. This rise in LH triggers the release of the mature egg from the dominant follicle.

The release of the mature egg happens as the egg inside the follicle detaches itself, and then ruptures through the walls of the follicle into the fallopian tube. The egg is picked up into the fallopian tube by small finger-like branches and then gently pushed down towards the uterus. On its way, the egg will either meet sperm and be fertilised or it will arrive in the uterus unfertilised.

The life span of the typical egg is only around 24 hours. Unless it meets a sperm during this time on its journey down the fallopian tube, it will be die and be absorbed back into the body.

Luteal phase

After ovulation has occurred the follicle transforms into a something known as the corpus luteum and begins to produce progesterone as well as estrogen. This combination of hormones keep the lining of the uterus thick in preparation for implantation of a fertilised egg. 

If a fertilised egg implants, it produces the hormones that are necessary to keep the corpus luteum active. One of these hormones is the human chorionic gonadotrophin (HCG), which is the hormone that is detected in a urine test for pregnancy. The corpus luteum keeps producing progesterone which is needed to maintain the thickened lining of the uterus.

If no fertilisation occurs, the corpus luteum will start to break down around 10 days after ovulation. This results in a drop in estrogen and progesterone levels, which causes menstruation. The luteal phase typically lasts about 14 days,

Secretory phase

During the luteal phase in the ovaries, the uterine lining works in sync and itself produces chemicals that will either support an early pregnancy or will prepare the lining to break down and shed.

During this phase, the uterine lining (endometrium) secretes many types of chemical messages, including prostaglandins. If there is an early pregnancy, the production of prostaglandin is inhibited. However, is there is no pregnancy the prostaglandins cause the uterus muscles to cramp thus triggering menstruation.

This process along with the drop in oestrogen and progesterone caused by the breakdown of the corpus luteum in the ovaries,  causes menstruation to begin and the whole cycle starts again.