The typical woman's body can undergo significant hormone changes in the days and weeks before menstruation. These changes generally begin anew after each menstrual period ends. Hormonal changes generally start with a gradual increase in estrogen levels leading up to ovulation, which normally occurs on about day 14 of the average 28-day menstrual cycle. As the newly released egg travels down the fallopian tube towards the uterus, estrogen levels continue to increase, causing the lining of the uterus to thicken. If conception occurs, the fertilized egg will normally implant itself in the thickened uterine lining and begin to develop into a fetus, but if conception does not occur, the body's estrogen levels decrease and the uterus sheds its lining in what is commonly known as a menstrual period.
The average menstrual cycle is believed to last about 28 days, though it is considered normal for the cycle to vary from 21 to 35 days in length. Most healthy women enjoy regular menstrual cycles, in which menstruation begins on about the same day each month. The physical changes that generally occur in the days just before menstruation can serve to alert most women of the onset of a menstrual period.
Even healthy women who experience normal menstruation are considered likely to suffer some uncomfortable physical and emotional symptoms in the days just before menstruation. These symptoms are often collectively known as premenstrual syndrome (PMS). They can include bloating, fatigue, diarrhea or constipation, mood swings, and irritability. Though physicians do not fully understand what causes PMS, they believe it is connected to rising hormone levels in the body just before menstruation.
The normal menstrual cycle typically begins with a gradual increase in the hormone estrogen, which helps to stimulate ovulation, or the release of an egg from an ovary. Most women are believed to ovulate at the midpoint of their cycle, typically about two weeks after the last menstrual period. Women are generally considered most fertile on the three days prior to, and on the day of, ovulation.
Rising estrogen levels in the body also cause the uterine lining to become thicker. This thickened uterine lining typically develops into a fetal placenta, if conception occurs. If conception does not occur, estrogen levels decrease and menstruation begins. Symptoms of PMS typically abate in the first few days of the menstrual period, possibly due to lowered estrogen levels in the woman's body.