During pregnancy and for some time after childbirth, menstruation is normally suspended; this state is known as amenorrhoea, i.e. absence of the menstrual cycle. If menstruation has not resumed, fertility is low during lactation. The average length of postpartum amenorrhoea is longer when certain breastfeeding practices are followed; this may be done intentionally as birth control.
In many women, various intense sensations brought about by the involved hormones and by cramping of the uterus can precede or accompany menstruation. Stronger sensations may include significant menstrual pain (dysmenorrhea), abdominal pain, migraine headaches, depression, emotional sensitivity, feeling bloated, changes in sex drive and nausea.
Dysmenorrhea, or severe uterine pain, is particularly common for adolescents and young women (one study found that 67.2% of girls aged 13–19 suffer from it). Breast swelling and discomfort may be caused by premenstrual water retention. Binge eating occurs in a minority of menstruating women.
This may be due to fluctuation in beta-endorphin levels. More severe symptoms may be classified as premenstrual dysphoric disorder (PMDD). The sensations experienced vary from woman to woman and from cycle to cycle.
Some women experience emotional disturbances associated with menstruation. These range from the irritability popularly associated with premenstrual syndrome (PMS), to tiredness, or "weepiness" (i.e. tears of emotional closeness). A similar range of emotional effects and mood swings is associated with pregnancy. The prevalence of PMS is estimated to be between 3% and 30%. Rarely, in individuals susceptible to psychotic episodes, menstruation may be a trigger (menstrual psychosis).
The normal menstrual flow follows a "crescendo-decrescendo" pattern; that is, it starts at a moderate level, increases somewhat, and then slowly tapers. Sudden heavy flows or amounts in excess of 80 mL (hypermenorrhea or menorrhagia) may stem from hormonal disturbance, uterine abnormalities, including uterine leiomyoma or cancer, and other causes. Doctors call the opposite phenomenon, of bleeding very little, hypomenorrhea.
The typical woman bleeds for two to seven days at the beginning of each menstrual cycle.
Prolonged bleeding (metrorrhagia, also meno-metrorrhagia) no longer shows a clear interval pattern. Dysfunctional uterine bleeding is hormonally caused bleeding abnormalities. Dysfunctional uterine bleeding typically occurs in premenopausal women who do not ovulate normally (i.e. are anovulatory).
All these bleeding abnormalities need medical attention; they may indicate hormone imbalances, uterine fibroids, or other problems. As pregnant patients may bleed, a pregnancy test forms part of the evaluation of abnormal bleeding.