Amenorrhea or amenorrhoea is the absence of a menstrual period in a female organism who has reached reproductive age. Physiological states of amenorrhoea are most commonly seen during pregnancy and lactation (breastfeeding). In humans, it is where a woman or girl who has reached reproductive age who is not on birth control does not menstruate.

Amenorrhoea is a symptom with many potential causes. Primary amenorrhea is defined as an absence of secondary sexual characteristics by age 13 with no menarche or normal secondary sexual characteristics but no menarche by 15 years of age. It may be caused by developmental problems, such as the congenital absence of the uterus, failure of the ovary to receive or maintain egg cells, or delay in pubertal development. Secondary amenorrhoea, ceasing of menstrual cycles after menarche, is defined as the absence of menses for three months in a woman with previously normal menstruation, or six months for women with a history of oligomenorrhoea. It is often caused by hormonal disturbances from the hypothalamus and the pituitary gland, premature menopause, intrauterine scar formation, or eating disorders.

Although amenorrhea has multiple potential causes, ultimately, it is the result of a hormonal imbalance or an anatomical abnormality.

Physiologically, menstruation is controlled by the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus. GnRH acts on the pituitary to stimulate the release of follicle stimulating hormone (FSH) and luteinizing hormone (LH). FSH and LH then act on the ovaries to stimulate the production of estrogen and progesterone which, respectively, control the proliferative and secretary phases of the menstrual cycle. Prolactin also influences the menstrual cycle as it suppresses the release of LH and FSH from the pituitary. Similarly, thyroid hormone also affects the menstrual cycle. Low levels of thyroid hormone stimulate the release of TRH from the hypothalamus, which in turn increases both TSH and prolactin release. This increase in prolactin suppresses the release of LH and FSH through a negative feedback mechanism. Amenorrhea can be caused by any mechanism that disrupts this hypothalamic-pituitary-ovarian axis, whether that it be by hormonal imbalance or by disruption of feedback mechanisms.

Amenorrhea is classified as either primary or secondary.

Primary amenorrhoea is the absence of menstruation in a woman by the age of 16. Females who have not reached menarche at 14 and who have no signs of secondary sexual characteristics (thelarche or pubarche) are also considered to have primary amenorrhea. Examples of amenorrhea include constitutional delay of puberty, Turner syndrome, and Mayer–Rokitansky–Küster–Hauser (MRKH) syndrome.

It produces the appearance of secondary sexual characteristics, which are the sprouting of pubic and armpit hair, development of the breasts, and a lack of definition in the female body structure, such as the waist and hips.

Secondary amenorrhoea is defined as the absence of menstruation for three months in a woman with a history of regular cyclic bleeding or six months in a woman with a history of irregular menstrual periods. Examples of secondary amenorrhea include hypothyroidism, hyperthyroidism, hyperprolactinemia, polycystic ovarian syndrome, primary ovarian insufficiency, and functional hypothalamic amenorrhea.