What is considered an irregular periods? Irregular menstruation describes anything that’s not a normal menstrual period. Some examples of an irregular period are: Periods that occur less than 21 days or more than 35 days apart. Not having a period for three months (or 90 days). Menstrual flow that’s much heavier or lighter than usual. Period bleeding that lasts longer than seven days. Periods that are accompanied by severe pain, cramping, nausea or vomiting. Bleeding or spotting that happens between periods.