Oligomenorrhea is menstrual periods occurring at intervals of greater than 35 days, with only four to nine periods in a year. Also, menstrual periods should have been regularly established previously before the development of infrequent flow. The duration of such events may vary.

Oligomenorrhea with a prevalence of 12-15.3% in different studies around the world, is one of the most common types of menstrual bleeding disorders. In recent decades, as a result of changes in life style, obesity, low physical activity, unhealthy nutrition, and emotional stress, the prevalence of amenorrhea (the absence of a menstrual period) and oligomenorrhea has increased considerably.

Endurance exercises such as running or swimming can affect the reproductive physiology of women athletes. Female runners, swimmers and ballet dancers menstruate infrequently in comparison to nonatheletic women of comparable age or not at all (amenorrhea). The degree of menstrual abnormality is directly proportional to the intensity of the exercise. 

Among several etiologic factors, polycystic ovarian disease (PCOD) is the most important underlying factor for oligomenorrhea. Oligomenorrhea can be also a result of eating disorders, thyroid disfunction, hyperprolactinemi or due to Asherman syndrome. 

Polycystic ovary syndrom

Polycystic ovary syndrome (PCOS) is one of the most common but heterogeneous endocrine metabolic disorders women of reproductive age and causes abnormal ovulation and infertility. The characteristic clinical features of PCOS include oligomenorrhea or amenorrhea, hyperandrogenism, and polycystic ovarian morphology. Excessive androgens (male sex hormones) are responsible for menstrual disorders. 

Eating disorders

Eating disorders can also result in oligomenorrhea. Although menstrual disorders are most strongly associated with anorexia nervosa, bulimia nervosa may also result in oligomenorrhea or amenorrhea. There is some controversy regarding the exact mechanism for the menstrual dysregulation, since amenorrhea may sometimes precede substantial weight loss in some anorexics; thus some researchers hypothesize that some as-yet unrecognized neuroendocrine phenomenon may be involved, and the menstrual irregularities may be related to the biological undergirding of the disorders, rather than a result of nutritional deficiencies.

Thyroid dysfunction

Thyroid dysfunction is extremely common in women, as women are five have thyroid dysfunction than men, and has unique consequences related to menstrual cyclicity and reproduction. Both hyperthyroidism (excessive production of thyroid hormone) and hypothyroidism (thyroid gland does not produce enough thyroid hormone) may result in menstrual disturbances. The most common manifestations are hypomenorrhea and oligomenorrhea. The connection between thyroid hormone levels and the menstrual cycle is mainly mediated by thyrotropin-releasing hormone (TRH), which has a direct effect on the ovary. Additionally, abnormal thyroid function can alter levels of sex hormone-binding globulin, prolactin, and gonadotropin-releasing hormone, contributing to menstrual dysfunction. For example, increased levels of TRH may raise prolactin levels, contributing to the amenorrhea associated with hypothyroidism.

Hyperprolactinemia

Hyperprolactinemia is known to cause amenorrhea, oligomenorrhea and galactorrhea ( the spontaneous flow of milk from the breast, unassociated with childbirth or nursing) in females and is also associated with sexual dysfunction and bone loss. The main physiologic function of prolactin is to cause breast enlargement during pregnancy and milk production during lactation and also cause disruption in menstrual cycles. The reductions in sexual functioning and fertility associated with nursing may have evolutionary advantages. Breast-feeding is not contraception, but women‘s fertility is decreased.

Asherman syndrome

Asherman syndrome is a condition characterized by adhesions and/or fibrosis of the endometrium particularly.
The cavity of the uterus is lined by the endometrium. This lining is composed of two layers, the functional layer (adjacent to the uterine cavity) which is shed during menstruation and an underlying basal layer (adjacent to the myometrium), which is necessary for regenerating the functional layer. Trauma to the basal layer can lead to the development of intrauterine scars resulting in adhesions that can obliterate the cavity to varying degrees. Even with relatively few scars, the endometrium may fail to respond to estrogen. Often, patients experience secondary menstrual irregularities characterized by a decrease in flow and duration of bleeding (amenorrhea, hypomenorrhea, or oligomenorrhea) and become infertile.

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