If your period is late, you might immediately think you are pregnant. But while pregnancy is certainly a common reason, this isn’t always the case. Amenorrhea is the medical term for a missed period or the absence of menstruation. Primary amenorrhea is when you are over the age of 15 and have not yet begun menstruation. Secondary amenorrhea is when you have been menstruating and suddenly stop.
Irregular periods - While many women follow a 28 day schedule, approximately one third of all women have irregular periods at some time in their reproductive lives. This means you may sometimes miss your period altogether or be late. Hormonal fluctuations are one reason for this, especially in young girls within the first few years of menstruating.
Weight Changes - Low body weight or excessively losing weight or losing weight quickly can cause you to miss your period. Obesity can also cause changes in your menstrual cycle.
Stress - High stress levels can interfere with ovulation and therefore interfere with your regular menstrual cycle. If ovulation is delayed, your period may be late. If you don’t ovulate at all, you may miss your period.
Contraceptives - Hormonal contraceptives can cause missed or irregular periods. Some contraceptives, such as Seasonique or Yaz) stop your period for several months or up to a year and others may change your flow so it seems as if you have missed your period.
Exercise - Strenuous and rigorous exercise can interfere with menstrual cycles. This may be caused by the stress on your body or low body fat.
Medications - Some medications will interfere with your menstrual cycle. If you are taking medication, check with your pharmacist to find out if missed periods can be a side effect.
Thyroid disease - Hypothyroidism, an underactive thyroid, can change how your body produces certain hormones which can cause you to miss a period or to stop altogether.
Breastfeeding - Hormones that produce lactation can sometimes interfere with your menstrual cycle or block ovulation which will cause you to stop menstruating.
Eating disorders - Just as when you suddenly lose weight or are underweight, eating disorders can interfere with your hormones, causing irregular or missed periods.
Shift Changes - If you suddenly switch from day shift to night, or vice versa, or have a job which requires you to continually switch shifts, you may notice a change in your menstrual cycle.
It is not unusual for women to miss a period, so, if this happens, try to relax. In most cases, your cycle will return to normal next month. Amenorrhea is not normally diagnosed until you have missed at least 3 periods. Treatment would be based on the underlying reason for the missed periods.
“Irregular Periods,” Date Unknown, Staff Writer, University of California Student Health Services
"Secondary Amenorrhea," Updated 2010, June 16, Updated by Susan Storck, A.D.A.M. Medical Encyclopedia