Menstrual bleeding or period, is the womb lining exiting the body. Amenorrhea is the absence of menstrual bleeding. Causes of an absence of menstruation could be due to menopause, pregnancy, breastfeeding, Health conditions, medications, lifestyle.
The term also applies to people who have not started their period by the age of 16, which is called primary amenorrhea. Secondary amenorrhea occurs in people who have missed 3 months of periods

Polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a hormonal disorder that affects 6–8 percent of women worldwide. Causes of PCOS range include:
irregular periods, excessive hair growth, difficulty getting pregnant, weight gain, acne.
PCOS may also lead to hyperandrogenemia, which is when a female has high levels of male hormones.
A 2017 study of 266 women with PCOS found that over 78 percent of them had hyperandrogenemia.
Hyperandrogenemia may affect menstrual cycles and lead to amenorrhea.

Birth control

Some birth control pills may cause missed periods or the complete absence of menstruation.

Pituitary gland disease

The pituitary gland releases the hormones that control the menstrual cycle.
Issues with the pituitary gland, including those below, may lead to amenorrhea: a pituitary gland tumor, postpartum necrosis, which is the premature death of cells in the pituitary gland after pregnancy
sarcoidosis, which is an inflammatory condition.
Radiation therapy may also affect cells in the pituitary gland and cause amenorrhea.

Nutritional deficiencies

Malnutrition may affect how the hypothalamus and pituitary gland function, which can lead to amenorrhea.

Stress

Stress can affect hormone levels in the body, and it is another cause of functional hypothalamic amenorrhea. People with this type of amenorrhea also have higher rates of depression and anxiety


https://medfort.com.ng/causes-of-an-absence-of-menstruation/