The absence of menstrual periods is referred to as amenorrhoea.
If a young person hasn't shown any signs of puberty (such as starting to grow breasts and dark pubic hair) by the age of 14 or hasn't started having periods by 16, they should see a doctor. For most, it's just late puberty, but it's important to be sure.
People who have regular monthly periods that stop for more than three months and people who have periods that come at odd times should see a doctor. Often, a serious cause isn't found. The most common cause of amenorrhoea is when the body’s hormones are disrupted, which can happen because of emotional stress, losing or gaining a lot of weight, or exercising too much (referred to as athletic amenorrhoea).
For a doctor to diagnose amenorrhoea, all other possible causes, including certain reproductive disorders such as polycystic ovarian syndrome (PCOS), need to be eliminated first.
Treatment for amenorrhoea depends on the cause and may include exercising less or, in some cases (if overweight), recommending weight loss. Starting hormone therapy (HT) may also be an option.