Oligomenorrhea

n. (Medicine) oligomenorrhoea, abnormally light menstrual periods, scanty blood flow in the menstrual cycle, infrequent menstrual period that occur at long intervals, irregular and infrequent menstrual period

Oligomenorrhea is the medical term for infrequent or light menstrual periods in women of child-bearing age. Oligomenorrhea can also be a result of prolactinomas (adenomas of the anterior pituitary). May also be caused by thyrotoxicosis and Graves disease. (The medical term for a total cessation of the menstrual period is amenorrhea while unusually heavy periods are called menorrhagia)
See more at Wikipedia.org...

Medical dictionaries define oligomenorrhea as infrequent or very light menstruation. [more]Oligomenorrhea - Community and Resources

Noun
1. abnormally light or infrequent menstruation
(hypernym) menstruation, menses, menstruum, catamenia, period, flow

Scant menstruation. Less menstrual blood flow than usual.