There are many things that cause a missed period, but the most common cause is pregnancy, according to the Palo Alto Medical Foundation. Although pregnancy is not the only thing that causes a girl to miss her period, if she is sexually active, pregnancy has to be considered. This is especially true if the girl or woman typically has a regular cycle.
The Palo Alto Medical Foundation states that girls should not panic if they do not get their period on time. Factors other than pregnancy affect regular menstrual cycles as well. For example, girls who participate in excessive exercise are likely to notice a change in their period. Sometimes they notice less frequent periods, and other times their period stops altogether. Most of the time, girls' periods return after several months of not engaging in excessive exercise.
Birth control pills also can cause women to miss their period, according to Palo Alto Medical Foundation. Although these pills are supposed to help regulate the menstrual cycle, there are reports of some women missing their periods altogether while taking birth control pills. It is not considered abnormal to miss a period when taking contraceptives. Periods return after discontinuing use of the birth control pills.