This is day two of your period and day two of your complete menstrual cycle, which starts on the first day of your period and ends on the first day of your next period. A full cycle is, on average, 28 days, but many women have a shorter or longer cycle.

This may be the time when your period is at its heaviest, since the tissue and blood that make up the lining of the uterus (the endometrium) is shed. The average blood loss during menstruation is around two tablespoons (30 ml). While the lining is being sloughed off, the blood vessels in the uterus constrict, which can cause cramplike period pains. As soon as your period has finished, an egg begins to mature within its follicle in one of your ovaries, ready to be released around mid-cycle. This is called ovulation (see This is Day 14 of your Menstrual Cycle).

Meanwhile, the lining of the uterus starts to build up again under the influence of the hormones progesterone and estrogen, ready to receive a fertilized egg. If the egg is not fertilized, hormone levels fall, the lining sheds, and the cycle begins again.

The monthly cycle of an egg as it grows to maturity inside an ovary is shown at the top of the chart. It is released from its follicle around day 14. The bottom of the chart shows the corresponding development of the lining of the uterus-shedding at the start of a period, then rebuilding in preparation for the fertilized egg.

The empty egg follicle (called the corpus luteum) secretes progesterone, which is a hormone that helps the endometrium reach a thickness of about 0.2 in (6 mm) over the 28 days of the menstrual cycle, ready to receive a fertilized egg.

Periods can synchronize in women who live or work together.

Scientists claim that pheromones (chemicals that trigger a biological response in someone) waft from one woman to another. Receptors in the nose detect these pheromones and a biological process takes place whereby one woman naturally adjusts her menstrual cycle.