The menstrual cycle is the time between the first day of a girl’s period till the first day of her next period.
This period can fluctuate from 3 to 6 weeks. The average cycle is 28 days.
The menstrual cycle can be very irregular for the first two years. It can take three weeks till your next period, but also half a year. After two years it becomes more regular.
Halfway a girl’s menstrual cycle ovulation takes place. This is the time she is most likely to get pregnant if she has intercourse without using a contraceptive.

What happens during a girl’s cycle?
How big is the chance of getting pregnant?