Regular or irregular? It's very natural for the cycle to be irregular (not occurring every month or for the same amount of time every month) when a woman first starts her period. It could take up to five years for a female to become regular. However, if one is sexually active and has a missed period, there is a possibility that she could be pregnant. If she is not sexually active, other factors such as diet and stress could be affecting the regularity of menstruation.

Not eating enough or exercising too much could cause a missed period. Also, being stressed out (ironically, for example, if one were stressed out about possibly being pregnant) could also cause a missed period. The best thing to do is to make sure to take care of yourself and relax. Eating a well balanced diet helps too.