During the menstrual cycle the uterine lining is prepared for the implantation of a fertilized ovum. If conception does not occur, the lining sloughs off and is discharged as menstrual flow. The length of the menstrual cycle is usually measured from the beginning of the first day of flow to the day before the next flow begins. The menstrual period itself typically lasts 2 to 6 days. It is normal for the volume of the menstrual flow (usually 6 to 8 ounces) to vary. The cycle length varies from woman to woman; it can be anywhere from 24 to 42 days.
Regardless of the total cycle length, the interval between ovulation and the onset of menstruation is 14 days, plus or minus 2 days, even when there is several weeks’ difference in the total length of the cycle, as shown in I Figure 3.5. Some women experience a twinge, cramp, or pressure in the lower abdomen, called Mittelschmerz (German for "middle pain"), at ovulation. Mittelschmerz is caused by the swelling and bursting of
Female Sexual Anatomy and Physiology
the follicle or by a little fluid or blood from the ruptured follicle that irritates the sensitive abdominal lining. The released ovum then travels to the fallopian tube. Occasionally, more than one ovum is released. If two ova are fertilized, nonidentical twins will develop. When one egg is fertilized and then divides into two separate zygotes, identical twins result.
Around the time of ovulation, secretions of cervical mucus increase because of increased levels of estrogen. The mucus also changes, becoming clear, slippery, and stretchy. The pH of this mucus is more alkaline; as noted earlier, a more alkaline vaginal environment contributes to sperm motility and longevity. This is the time in the cycle when a woman can most easily become pregnant.