First, it is important to realize that in most young girls the age of your daughter, it is perfectly normal to having missed or irregular periods. It is a common misconception that periods should be normal like they are in adult women right from the beginning. In fact, however, in the first few years after menstruation begins, your daughter's hormonal system is still developing and maturing, so it can be totally normal to miss periods or to have variation in the interval between periods. This irregularity should gradual normalize over months to years and does not usually need to be treated or evaluated.
Of course, as you mention, you should always think about pregnancy as well. Young teenagers are often sexually active, even when we think they are so young that this couldn't be a possibility. Finally, significant weight loss
and eating disorders can also be associated with irregular or missed periods. However, given your daughter's age and your lack of any specific concerns about these other possibilities, it is likely that this is just her normal development.
As always if you have additional questions about your daughter's development make sure to talk to her pediatrician