What are something's other than pregnancy that can cause a missed period?

Amenorrhea: The menstrual cycle is susceptible to outside influences, so missing a single menstrual period is rarely significant. If you have absent menses for more than 3 cycles or 6 months, pregnancy is the most common cause. Once this is ruled out, disorders of the hypothalamus, pituitary gland, thyroid, ovaries, and uterus should be considered.