###Human: What causes irregular periods in a 21-year-old? ###Assistant: Hello. At the age of 21 years, possibilities for such irregular cycles are, 1. You have severe physical or mental stress in any form, which would interfere with regular normal ovulation and periods will be haphazard. Try to reduce stress with meditation, breaks, etc. 2. If you have suddenly put on weight. This excess cholesterol will get converted into hormones and interfere with ovulation. So have a healthy lifestyle and diet. You can have cyclical birth control pills for five to six cycles. You have to take pills from day one of the periods for three weeks, then no pills for one week. After eight days start a new pack. Do not wait for next period. 3. You have not mentioned about your sexual activity. If sexually active, then infection could be a cause. If so, you and your partner need to meet a doctor and have a course of antibiotics. 4. If none of these a reason, have a pelvic scan to look for any local problem like polyp or fibroids inside the uterus. 5. Do have a serum thyroid hormone profile and serum prolactin levels to rule out, a hormonal angle. I hope I have clarified your query and do write back if any other questions, and for feedback.