A woman’s cycle begins with the first day of true bleeding in her period. Some women experience a few days of premenstrual spotting, in which a few drops of pinkish or brownish blood are visible when they wipe after urinating, but “Day 1” is considered to be the first day on which fresh, red blood appears, or when bleeding of any color becomes heavy enough to require protection. The menstrual period typically lasts for about 3-5 days, and during this time the old endometrial lining of the uterus is shed.
At about the same time menstruation begins, the ovaries begin to prepare to release the next egg. Hormones from the pituitary gland in the brain stimulate the ovarian follicles, and a few of them begin to grow and mature into eggs. As they grow, the ovaries begin to secrete more estrogen, which directs the uterus and cervix to prepare for ovulation. The cervix (the narrow opening at the bottom of the uterus) is tightly plugged with thick mucus after menstruation stops in order to protect the uterus from infection, and the vagina, while always moist inside, remains relatively dry, with little or no moisture escaping from the vaginal opening. When the developing eggs are about 5 days from release, however, the rising estrogen levels trigger a breakdown in the cervical mucus plug and cause the cervix to soften and dilate slightly, while the cervical glands produce thinner, more liquid mucus that lubricates the vagina and coats the vaginal opening. If a woman is paying attention, she can detect the change in sensation from dryness to dampness as she goes about her daily activities and when she urinates. As ovulation grows nearer, the mucus gradually changes from watery or milky to a slippery, stretchy, clear substance that resembles raw egg white; the woman will feel the change from damp to slippery, and the mucus will become increasingly apparent in the bathroom.
This mucus is crucial to a woman’s fertility. Not only does it make sexual intercourse easier and more pleasant, it helps support and guide sperm from the vagina up through the cervix and into the uterus and Fallopian tubes, where fertilization must take place. When no cervical mucus is present, sperm will die within a few hours of intercourse and be swept out of the vagina; with the help of fertile cervical mucus, sperm can live for 3-5 days and occasionally even longer. Since this mucus is typically produced for approximately 5 days immediately preceding ovulation, intercourse at any time when fertile-type mucus is present can lead to pregnancy.
As the follicles mature, typically all but one of them will gradually stop developing, leaving only one (or occasionally two) to reach full maturity in a given cycle. A final burst of hormones from the pituitary gland triggers the release of the mature egg from the ovary, from whence it will make its way into the Fallopian tube and begin traveling toward the uterus, a journey that takes about a week to complete. The egg lives for only about 12-24 hours after it is released. If sperm are present in the Fallopian tube during this time, fertilization may occur, in which case the fertilized egg will begin to divide and develop into an embryo as it moves toward the uterus, implanting in the uterine lining about a week after ovulation. If fertilization does not occur or is not successful, the egg will die and be reabsorbed or passed out of the body. Either way, the uterus will not know whether or not a baby is on the way until at least a week after ovulation, so it will prepare for pregnancy in the same way every month.
Since conception is no longer possible more than 24 hours after ovulation (the egg is either already fertilized or already dead), the hormone surge of ovulation also triggers the cervix to stop producing fertile mucus and to seal itself tightly shut, blocking the opening with a fresh plug of thick mucus. At this time, the woman will notice the sensations at the opening of the vagina changing abruptly (overnight or over the course of a day) from slippery to dry or damp. The last day on which a slippery sensation is observed is called the “Peak day” and marks the peak of her ovulation hormones; ovulation itself may occur on this day or on the next day (the first dry day) that follows it, or occasionally on the second day after Peak. Sexual intercourse on Peak day or on either of the two days that follow it may therefore lead to pregnancy; if ovulation occurs on the second day post-peak and the egg lives a full 24 hours, sex on the third day post-peak may occasionally lead to pregnancy as well. From the fourth day post-Peak until the end of the cycle, conception is extremely unlikely.
The period of time between ovulation and the beginning of the next menstrual period is called the Luteal Phase, because it is controlled by hormones produced by the corpus luteum, a structure in the ovary that develops out of the follicle from which the egg was released. The main hormone produced by the corpus luteum is progesterone, which stimulates the uterus to grow a thick, blood-rich lining into which an embryo can implant in case of pregnancy. In a healthy, fertile cycle, this lining will be ready to receive a developing embryo by about a week after ovulation and will remain in place until at least 11 days after ovulation, by which point implantation (if it occurs) should be complete. The corpus luteum will begin to break down naturally after about 11-16 days, and the resulting falling progesterone levels cause the uterine lining to break down and the cervix to open slightly, allowing the lining to be shed as menstrual flow. If implantation of an embryo does occur, hormones from the embryo will prevent the corpus luteum from breaking down and it will continue to release progesterone to support the pregnancy. During the luteal phase, a woman may observe a variety of sensations at the vaginal opening; typically it will feel dry or damp, but should not feel slippery as it did during the fertile phase.
A typical full cycle lasts about 28 days, the first 3-5 of which are usually occupied by the menstrual period. The end of menstrual bleeding is usually followed by several days to a week of dryness or an unchanging pattern of light, non-slippery discharge which is produced by the vagina and is not fertile. Fertile mucus production usually begins around cycle day 8-10 (counting Day 1 as the first day of menstrual bleeding), and ovulation occurs around day 14. The remainder of the cycle is typically characterized by dryness or dampness, with the next menstrual cycle beginning about 28 days after the last one. However, this pattern may vary from woman to woman or from month to month.
In an irregular cycle, it is usually the follicular phase (menstruation to ovulation) that varies. The ovarian follicles may mature quickly, with fertile mucus production (and therefore the potential for pregnancy if sexual intercourse occurs) beginning just a few days after menstruation begins, even while the woman is still bleeding. Alternatively, follicle maturation may be delayed and the fertile period may not begin for several weeks after menstruation. This can be caused by many factors, from hormonal imbalances that must be addressed by a gynecologist to simple, temporary problems like stress or illness. Even a perfectly healthy woman with perfect fertility may occasionally experience a delayed ovulation during times of high stress or if she becomes ill during the pre-ovulatory phase, even if fertile mucus production has already begun. If this is the case, the mucus may dry up temporarily without reaching a true Peak, or mucus production may simply continue for a few extra days before proceeding to a Peak. Therefore, it is important to learn to recognize a real Peak pattern and be alert to signs that ovulation has not yet occurred. A true Peak is usually characterized by a minimum of three days of fertile-type mucus that displays a changing, developing pattern ending in a slippery sensation, changing abruptly to a sensation of dryness or dampness. If ovulation is delayed by illness or stress and mucus production stops, it will usually resume within a few days of recovery.
The luteal phase (ovulation to menstruation) is usually highly consistent from month to month, though it may vary in length from woman to woman—one woman may have an LP of 11-12 days each month, for instance, while a different woman may have an LP of 14-15 days per month. 14 days is about average for a healthy, fertile woman. Thus, no matter when ovulation is observed, it is possible to predict with a high degree of accuracy when menstruation will begin. During the first cycle of charting, a woman can assume it will begin about 14 days after Peak; in subsequent cycles, she should predict that it will start about the same number of days after Peak as it did before, and her predictions will grow more accurate as she continues to chart. Luteal phase length may, however, vary if a woman has recently given birth and her fertility cycles have not yet stabilized, if she is approaching menopause, or if she has certain hormonal disorders. A luteal phase that is shorter than 11 days in length may cause difficulty in becoming pregnant, as the uterine lining begins to break down before implantation of the embryo is complete