Menstrual cycle

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

Girls get their first period (menarche), on average, between the ages of 11 and 14. By this stage, other sexual characteristics have usually developed, such as pubic hair and budding breasts.

Hormones and the menstrual cycle

The menstrual cycle is complex and controlled by many different glands and the hormones they produce. A brain structure called the hypothalamus causes the nearby pituitary gland to produce certain chemicals, which prompt the ovaries to produce the sex hormones oestrogen and progesterone.

The menstrual cycle is a biofeedback system, which means each structure and gland is affected by the activity of the others. The four main phases of the menstrual cycle are menstruation, the follicular phase, ovulation and the luteal phase.

Menstruation

Menstruation is the elimination of the thickened lining of the uterus (endometrium) from the body through the vagina. Menstrual fluid contains blood, cells from the lining of the uterus (endometrial cells) and mucus. The average length of a period is between three days and one week, depending on the individual.

Sanitary pads or tampons are used to absorb the menstrual flow. Both pads and tampons need to be changed regularly (at least every four hours). Using tampons has been associated with an increased risk of a rare illness called toxic shock syndrome.

Follicular phase

The follicular phase starts on the first day of menstruation and ends with ovulation. Prompted by the hypothalamus, the pituitary gland releases follicle stimulating hormone (FSH). This hormone stimulates the ovary to produce around five to 20 follicles (tiny nodules or cysts), which bead on the surface.

Each follicle houses an immature egg. Usually, only one follicle will mature into an egg, while the others die. This can happen at around day 10 of a 28-day cycle. The growth of the follicles stimulates the lining of the uterus to thicken in preparation for possible pregnancy.

Ovulation

Ovulation means the release of a mature egg from the ovary’s surface. This generally happens at mid-cycle, around two weeks or so before menstruation starts. During the follicular phase, the developing follicle causes a rise in the level of oestrogen. The hypothalamus in the brain recognises these rising levels and releases a chemical called gonadotrophin-releasing hormone (GnRH). This hormone prompts the pituitary gland to produce raised levels of luteinising hormone (LH) and FSH.

Within two days, ovulation is triggered by the high levels of LH. The egg is funnelled into the fallopian tube and towards the uterus by waves of small, hair-like projections. The life span of the typical egg is only around 24 hours. Unless it meets a sperm during this time, it will die.

Luteal phase

During ovulation, the egg bursts from its follicle, but the ruptured follicle stays on the surface of the ovary. For the next two weeks or so, the follicle transforms into a structure known as the corpus luteum. This structure starts releasing progesterone, along with small amounts of oestrogen. This combination of hormones maintains the thickened lining of the uterus, awaiting a fertilised egg to stick (implant).

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

If pregnancy doesn’t happen, the corpus luteum withers and dies, usually around day 22 in a 28-day cycle. The drop in progesterone levels causes the lining of the uterus to fall away. This is known as menstruation. The cycle then repeats.

Common menstrual problems

Some of the more common menstrual problems include:

• Premenstrual syndrome (PMS) - hormonal events before a period can trigger a range of side effects in women at risk, including fluid retention, headaches, fatigue and irritability. Treatment options include exercise and dietary changes.
• Dysmenorrhoea - or painful periods. It is thought that the uterus is prompted by certain hormones to squeeze harder than necessary to dislodge its lining. Treatment options include painkillers and the oral contraceptive pill.
• Menorrhagia - or heavy menstrual flow. If left untreated, this can cause anaemia. Treatment options include oral contraceptives to regulate the flow.
• Amenorrhoea - or absence of menstrual periods. This is considered abnormal, apart from during pre-puberty, pregnancy, lactation and menopause. Possible causes include low or high body weight and excessive exercise.

Where to get help

• Your doctor
• Gynaecologist
• Family Planning Victoria Tel. 1800 013 952 or (03) 9257 0100.

Things to remember

• The menstrual cycle is complex and controlled by many different glands and the hormones they produce.
• The four phases of the menstrual cycle are menstruation, the follicular phase, ovulation and the luteal phase.
• Common menstrual problems include heavy or painful periods and premenstrual syndrome.

You might also be interested in:

• Headache and hormones.
  
• Menopause.
  
• Menstruation - abnormal bleeding.
  
• Menstruation - amenorrhoea.
  
• Menstruation - athletic amenorrhoea.
  
• Menstruation - dysmenorrhoea.
  
• Ovulation.
  
• Ovulation pain.
  
• Pregnancy - signs and symptoms.
  
• Premenstrual syndrome (PMS).
  
• Puberty.
  
• Reproductive system.
  
• Vaginal bleeding - irregular.
  

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Family Planning Victoria

Fact sheet currently being reviewed.
Last reviewed: May 2011

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The four phases of menstrual cycle are menstruation, the follicular phase, ovulation and the luteal phase. Common problems include heavy or painful periods and premenstrual syndrome.

Content on this website is provided for education and information purposes only. Information about a therapy, service, product or treatment does not imply endorsement and is not intended to replace advice from your qualified health professional. Content has been prepared for Victorian residence and wider Australian audiences, and was accurate at the time of publication. Readers should note that over time currency and completeness of the information may change. All users are urged to always seek advice from a qualified health care professional for diagnosis and answers to their medical questions.

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