Irregular menstruation is a menstrual disorder whose manifestations include irregular cycle lengths as well as metrorrhagia (vaginal bleeding between expected periods). The possible causes of irregular menstruation may vary. The common factors of it are related to lifestyle, such as stress, body weight, and smoking status. Several studies indicate that COVID-19 vaccine of any type may disrupt the menstrual cycle, although only momentarily. This side effect should resolve on its own in the following month.

Irregular cycles or periods

Irregular cycles or irregular periods is an abnormal variation in length of menstrual cycles. An individual usually experiences cycle length variations of up to eight days between the shortest and longest cycle lengths. Lengths ranging between eight and 20 days are considered moderately irregular. Variation of 21 days or more is considered very irregular. Alternatively, a single menstruation cycle may be defined as irregular if it is less than 24 days or more than 38 days. If they are regularly shorter than 21 days or longer than 36 (or 35) days, the condition is termed polymenorrhea or oligomenorrhea, respectively. Additionally, irregular menstruation is common in adolescence. A regular menstrual cycle can be set within a year of menarche. However, other studies suggest that it can take anywhere between 2 and 7 years to establish regularity after an individual's first menses.

Other types

Other types of conditions that can be referred to by "irregular menstruation" include: