Oligomenorrhea is defined as an irregular mensuration cycle. More specifically, oligomenorrhea occurs when a women that has had regular periods, starts to experience atypical and infrequent periods. The usual amount of days between regular periods are 21-35 days. Women with oligomenorrhea experience periods with more than 35 days in between, and have fewer than 9 periods annually. Having some variations or irregularity within menstrual cycles is normal, as long as they return to some-what normal eventually. If periods have not gone back to normal after some time, that's when seeking a medical option is best to determine the sudden change. 

Symptoms of Oligomenorrhea:

• More than 35 days without a period
• < 9 periods in year
• Irregular mensuration
• Lighter periods

Causes of oligomenorrhea include: Beginning of mensuration for a young girl, periomenopause, pregnancy/breastfeeding. Other causes include: increased amount of stress, weight loss, intense exercise (frequently in athletes), being overweight/underweight (malnutrition), certain medications - birth control, anticoagulants, epilepsy treatments, & anti-anxiety or antipsychotic medication, and having an underlying health condition(s).

Treatments for Oligomenorrhea:

• Hormonal birth control or hormone therapy (gonadotropin-releasing hormone agonists
• Lifestyle changes - Resources to manage/cope with stress, or to help gain weight if need be
• Treatment based on underlying health condition

Reference URL's:

https://www.verywellhealth.com/an-overview-of-oligomenorrhea-4584379

http://www.healthofchildren.com/N-O/Oligomenorrhea.html

https://www.verywellhealth.com/bleeding-between-periods-3520522

https://www.verywellhealth.com/infertility-and-thyroid-disease-4019413