Few women have completely regular periods. Most menstrual cycles are between 21 and 35 days long, with 28 days the norm, but only 10 to 15 percent of women have regular 28 day cycles, according to the Merck Manual. Many conditions can delay a menstrual period from starting during the usual time period.
Anovulation
Anovulation means that a woman doesn't ovulate or release an egg. Anovulation often leads to amenorrhea, no menstrual periods or long intervals between periods. When bleeding does occur, it often proves very heavy due to the excessive build-up of the uterine lining. Polycystic ovarian syndrome, also known as PCOS, often causes anovulation. PCOS, a hormonal imbalance caused by an excess of androgens, male hormones, leads to irregular menstrual cycles and often infertility, the American Congress of Obstetricians and Gynecologists reports.
Delayed Ovulation
Ovulating later than normal during a menstrual cycle can also cause a delayed period. The time between ovulation and the start of a period is fairly constant, lasting normally around 14 days, the Advanced Fertility Center of Chicago states. If a woman ovulates later in the cycle than normal, her period will become delayed. Stress, weight gain or loss, new medications, illness, strenuous exercise or a change in routine or schedules can all cause late ovulation.
Perimenopause of Menopause
The time before menopause begins, called perimenopause, can last anywhere from 2 to 8 years, states the Mayo Clinic. During perimenopause, menstrual cycles often become irregular, occurring closer together if a woman ovulates very early in the cycle or prolonged if she ovulates late or not at all. Perimenopause normally begins in a woman's 40s, but early symptoms can start when she's in her 30s. Menopause, the cessation of menstrual periods for 12 months, generally doesn't occur suddenly, the University of Maryland Medical Center explains.
Pregnancy
The most common cause of a delayed period in women of childbearing age is pregnancy. The menstrual period normally starts when estrogen and progesterone levels drop as the corpus luteum, the shell of the follicle that contained an egg, deteriorates if pregnancy doesn't occur. The uterine lining breaks down and sheds as menstrual flow. If a woman becomes pregnant, estrogen and progesterone levels remain high and the uterine lining remains intact.
