polymenorrhoea BrE
polymenorrhea AmE

Polymenorrhea is the medical term for cycles with intervals of 21 days or fewer.

Some women with PCOS have polymenorrhea, or periods that are too frequent.

Regular cycles with intervals of 21 days or fewer are polymenorrhea; frequent but irregular menstruation is known as metrorrhagia.

However, if they are regularly shorter than 21 days, or longer than 36 (or 35) days, the condition would rather be termed polymenorrhea or oligomenorrhea, respectively.