Underweight is characterized by unhealthy low body weight. To define the weight status of a person, body mass index (BMI) is used. BMI put in relation the weight and height of a person (BMI= weight(kg)/height (m)2) and it approximately reflects the level of body fat in individuals. Underweight refers to people with BMI under 18,5 or weight 15 % - 20 % below normal in relevance to age and height group. A person may be underweight due to genetics, metabolism, drug use, lack of food (frequently due to poverty), or illness.

Underweight might be secondary to malnutrition or symptomatic of an underlying disease. Unexplained weight loss may require professional medical diagnosis. Being underweight is associated with certain medical conditions including hyperthyroidism (abnormally increased function of thyroid gland), cancer, or tuberculosis. People with gastrointestinal or liver problems may be unable to absorb nutrients adequately. Malnourishment can also cause anaemia (low red blood cell counts) and hair loss. Severely underweight individuals may have poor physical stamina and a weak immune system. Being underweight is an established risk factor for osteoporosis (reduce strength of bones), even for young people. Being underweight is also reported to increase mortality in smokers. People with certain eating disorders (anorexia nervosa, bulimia nervosa) can also be underweight due to lack of nutrients/over exercise. In women, being severely underweight as a result of an eating disorder, or due to excessive strenuous exercise can result in amenorrhea (absence of menstruation), infertility and, if gestational weight gain is too low, possible complications during pregnancy.

Body mass index (BMI) and weight are closely related to reproductive function. Underweight leads to increased time of conception and also increases the chance of pre-term birth. For women, being underweight and having extremely low amounts of body fat are associated with ovarian dysfunction. Additionally, the risk of ovulatory infertility increases in women with a BMI below 17. As women with elevated BMI usually require less hormonal stimulation during IVF (in vitro fertilization) cycles, being underweight may complicate this procedure. Men who are underweight are also at risk of infertility. Underweight in men tend to cause lower sperm concentrations than those who are at a normal BMI. 

Ovarian dysfunction (polycystic ovary syndrome, amenorrhea, hyperprolactinemia) 

Amenorrhea is the absence or abnormal cessation of the menses in a woman of reproductive age. Amenorrhea is part of so called ovarian dysfunction which is a term that also include disorders like polycystic ovary syndrome or hyperprolactinemia (high concentrations of luteotropic hormone – prolactin - in blood). The common feature of listed disorders is that all of them may be caused by hormonal disturbances. The underweight is united with significant decrease of oestrogen (sex hormone) production due to caloric intake or excessive exercise. As oestrogen is necessary for proper function of reproductive organs, its low levels also affects the sex gland (ovary) resulting in reduced function or even stoppage of the functionality.

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Underweight ―sourced from Wikipedia licensed under CC BY-SA 3.0
Female infertility of endocrine origin ―by Weiss and Clapauch licensed under CC BY-NC 3.0
Infertility ―sourced from Boundless licensed under CC BY-SA 4.0
AMENORRHOEA ―sourced from Fertilitypedia.org licensed under CC BY-SA 4.0
POLYCYSTIC OVARY SYNDROME ―sourced from Fertilitypedia.org licensed under CC BY-SA 4.0
ANOREXIA NERVOSA ―sourced from Fertilitypedia.org licensed under CC BY-SA 4.0
Body Mass Index (BMI) Measurement ―sourced from Wikispaces licensed under CC BY-SA 3.0
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