Most girls get their first periods around age 12, but some may begin as early as age 8, notes Mayo Clinic. Doctors cannot predict with certainty when girls begin menstruation, but it generally happens between two and three years after they start developing breasts.
It is common for a first period to be light, with only spotting or a few drops of blood, and for periods to be irregular for the first few years, states Mayo Clinic. Girls' menstrual cycles can vary from between 21 and 45 days and are usually longer for the first few years. Once a girl begins menstruating, it is physically possible for her to become pregnant, as the body begins releasing eggs through ovulation. If an egg remains unfertilized, the body sheds the lining of the uterus it had prepared for pregnancy, resulting in menstruation.
Parents should take their daughters to doctors if they do not get their first periods by age 15 or within three years after they started developing breasts, according to Mayo Clinic. Once their menstrual cycles have begun, girls should see their doctors if their pain doesn't respond to over-the-counter medications, they bleed between periods, their periods become irregular after being regular or their menstrual cycles last more than seven days.