Astrology explains this for better than modern science. Normally close friends have similar sun, moon positions in their horoscopes. The moon and mars determine the menstrual cycles. Astrological harmony makes them friends than living together which in turn is the cause of synchronization. Even if they are living far in different continents they will be in sync. In India any normal astrologer can check a woman’s horoscope and can tell the date of her menstruation time ( especially used when they have to attend auspicious ceremonies) and plan the events. (errors in the original)