Oligomenorrhoea
Infrequent periods, defined as cycles longer than 35 days or fewer than about nine a year. A common cycle pattern in PMOS.
Oligomenorrhoea means infrequent menstrual periods. In practice it is usually defined as cycles longer than 35 days, or fewer than about nine periods in a year. (When periods stop for longer stretches, the term is amenorrhoea.)
In PMOS, infrequent cycles usually reflect ovulation that is not happening on a regular schedule. Follicle maturation stalls, ovulation is skipped or delayed, and the cycle stretches out or becomes unpredictable. This is the cycle pattern that, alongside hyperandrogenism and ovary morphology, makes up the Rotterdam criteria.
One nuance: in the first year or two after periods begin, irregular cycles can be normal developmentally, which is why adolescent diagnosis uses different criteria. For adults, a persistent oligomenorrhoeic pattern is worth raising with a clinician.