The official medical term for the phenomenon that you're describing is "menstrual synchrony." When girls or women live together in close proximity, such as in a dormitory, they often find that their periods all start at around the same time each month. It is likely that this is caused by pheromones, or chemicals, that are constantly being released from our bodies. Though you cannot see or smell these chemicals, they can actually be detected by others' bodies and cause subtle hormonal shifts. When women live together and therefore spend a great deal of time with one another, they are constantly exposed to each other's pheromones. Over time, this can influence the timing of their menstrual cycles.
Every time this topic comes up I am in awe of the number of women who say this is true. From my understanding, girls secrete hormones from their glands that we instinctively pick up on, which may allow us to cycle together. This doesn't always happen. For instance, my daughter and I don't cycle together, though we are both on birth control pills that regulate our periods. In college, lots of girls seemed to cycle together the longer they hung out with one another. But it's not an exact science. It doesn't happen to every group of girls.
This is a highly debated subject and although many say this is a myth, while in college my roommates and I experienced a shift in our cycles that resulted in…synchronized periods!
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