Irregular periods are caused by a variety of factors, including age, lifestyle changes and disorders involving the reproductive organs. Medicines, such as those for birth control, can cause a woman to have lighter or more infrequent periods, according to WebMD.
Women who are just beginning or are nearing the end of their reproductive years may experience irregular periods, according to WebMD. At the beginning, it can take a few years for hormones to level out and produce a regular menstrual cycle. At the end, some women experience irregular periods during perimenopause as the ovaries begin to decrease estrogen production.
Lifestyle factors also affect a menstrual cycle. Women who gain a lot of weight or who are struggling with an eating disorder may have irregular periods, according to WebMD. Exercising heavily can change the regularity of a period as seen in endurance athletes. Stress, illegal drug use and recent illness are all factors as well.
Disorders involving the reproductive organs can cause a women to have irregular periods. One such disorder, polycystic ovary syndrome or PCOS, is common among women of reproductive age, according to Mayo Clinic. Aside from irregular periods, other symptoms of PCOS include excess body hair growth, acne and male-pattern balding due to elevated levels of male hormones.