It is a popular belief that women who live/spend alot of time together synchronise their menstrual cycles.

One of the earliest examples is a 1971 study widely credited with “proving” that women who live together or are close friends tend to be on a similar cycle and attributed this to chemical signals given off by the body known as pheromones. However, the study’s statistical design was criticized because while a few researchers were able to replicate the results in future studies, other researchers were unable to replicate them. Even though the study itself had holes, the myth has persisted.

Even modern research now states that if women happen to be on the same cycle at the same time, it's more likely coincidence and random, rather than scientific. Given that there are so many factors involved in the length of a woman’s cycle and her fertility (e.g., miscarriages, pregnancy, postnatal, stress, exercise activity, weight loss or gain, and nutrition), it’s easy to see that there is a great deal of variability in an individual woman’s cycle that would make it difficult for her menstrual cycle to synchronize consistently with the women she is in close contact with.

Conclusion?

It turns out with more research findings, our cycles do not sync up with the people we're in close proximity with and if they do, they are purely coincidental. Our bodies are as complex as our menstrual cycles. 

Fret not, there's tons of other things we can bond with our girls with!