Menstrual cycle

The menstrual cycle is calculated from the first day of your period. Typically, the menstrual cycle lasts 28 days. It varies from woman to woman, however, so may be shorter or longer. It is not always regular.

Periods are a flow of blood. Following a drop in hormone levels, the mucous lining of the uterus (the endometrium) is shed and expelled along with the menstrual blood. Periods tend to last between 3 and 6 days.

Phases of the menstrual cycle

There are two distinct phases of the menstrual cycle.

1. Pre-ovulation phase
   The pre-ovulation phase comprises two sub-phases:
   1. Menstrual phase
      The menstrual phase lasts from day 1 to day 5. Periods are caused by a drop in hormone levels, which leads to changes in the uterus' mucous lining (endometrium) and the flow of blood.
   2. Follicular phase
      The follicular phase runs from day 6 to day 14 of the cycle. During this phase, the uterine mucous thickens, stimulated by hormones: progesterone and oestrogen.
   3. Post-ovulation phase
      The post-ovulation phase runs from day 15 to day 28 of the cycle. This is when a fertilised egg can implant in the uterus. If fertilisation does not occur, the hormone levels drop, leading the mucous to deteriorate.

The end of the cycle is marked by the occurrence of your period, and the next cycle begins on the first day of your period.

Ovulation and fertilisation

From birth, girls have ovocytes (immature eggs) in their ovaries. During the follicular phase of the menstrual cycle, one of these ovocytes grows within a follicle. The follicle is a small sac in the ovary in which ova (eggs) develop.

Ovulation generally occurs on the 14th day of the menstrual cycle. The follicle ruptures to release the ovum. The egg then leaves the ovary and travels through the Fallopian tube to the uterus. Once it is released, the ovum can only survive between 12 and 24 hours.

Fertilisation of the ovum can only take place during the ovulation window, which runs between the 11th and 16th days of the cycle.

If the ovum is fertilised by a spermatozoon (sperm cell), this fertilisation takes place in the Fallopian tube.

The fertilised egg then travels down the Fallopian tube into the uterus, where it implants. This is the start of pregnancy.

If fertilisation does not occur, the cycle ends at around the 28th day, and a new cycle begins on day 1 of your period.

These cycles are not always absolutely regular, and the ovulation window may be before or after day 14 of the cycle. Sometimes, you may even have cycles where no ovulation takes place.