If you are around another girl who is on her period, does it mess up your cycle where you get it too?

Menstrual synchrony (when girls/women get their periods at the same time) is actually a myth. In the early 1970’s there was a study done, later called the “McClintock Effect” that suggested that women who lived together such as in college dorms, often had their periods at the same time. Since that time, other research studies have shown that girls/women who live together do not necessarily get their periods at the same time. With that being said, you may find at times that you and your roommate or sister may have your periods at the same time, however there is no scientific explanation for this—rather it is a coincidence.