The menstrual cycle

What is the menstrual cycle?

Most girls have their first period (menstruation) between the ages of 12 and 14.
Most women have a period at regular intervals of 28 days. However, the gap between two periods can be shorter or longer. At first the intervals are often irregular, but this usually settles down by itself.

What happens during a cycle?

The menstrual cycle starts on the first day of bleeding. The brain releases hormones that stimulate several eggs in the ovaries to mature. The ovaries contain about 1 to 2 million immature eggs. Within the first 14 days, some of these eggs mature inside their follicles.

The hormones produced in the follicle (oestrogens) make sure that the lining of the uterus grows. The oestrogens released as the follicles grow also produce a signal that triggers ovulation.
The follicle stays in the ovary and forms the corpus luteum, which is important for producing progesterone during the second half of the cycle.
So, over the next 14 days, the lining of the uterus is ready for the fertilised egg to implant, and no new egg matures.
After ovulation (6–8 hours), the egg can be fertilised by a sperm, and the now fertilised egg travels towards the uterus.
If the egg is not fertilised, it breaks down after 24 hours. The lining of the uterus, which had been prepared to receive the fertilised egg, is shed. The period begins and a new menstrual cycle starts.