It is often dismissed as a myth or just something silly but it is in fact a part of evolution.
Women that live together or spend a lot of time together often find that their menstrual cycles move closer together, mood swings are closer together and therefore often pregnancies are also at the same times.
The reasons for this dates back to when humans were cave dwellers at this time the men would have to go out and hunt in packs for safety and if all the women were having their periods at the same time the men were more likely to want to go out and hunt together than stay home and have sex.
Another reason for the synchronisation is the survival of the species. At the time it was very difficult to live very long, lots of things could easily kill humans particularly woman who were weaker than the men. Because of this woman would often have babies at the same time as other women living in close proximity would be able to feed the child if the mother died either in child birth or from other causes, also the children would grow up very close in age and develop sexual maturity at the same time allowing for more pack hunting and mating.
Although now these things are no longer necessary, the synchronisation still exists. Women give off pheromones when they sweat these smells change very subtly at every stage of a woman cycle. When women are close together they smell each others pheromones and their cycles adapt accordingly. This has been proved in studies where women where subjected to each other’s scent every day and their menstruation cycles synchronised despite having never actually met each other.