Now we want to back up for a minute and break down the female reproductive cycle in a bit more detail. A woman’s menstrual cycle is typically divided into two phases—follicular and luteal.
For our purposes, however, there are four significant phases to the cycle, and as you get into the Making Babies program, you’ll find that in many instances our advice changes for each phase. We’re going to tell you a bit about each phase here—first from a Western and then from an Eastern perspective—so that you’ll have a basic understanding of the mechanics underlying the practical applications to come.
For our purposes, however, there are four significant phases to the cycle, and as you get into the Making Babies program, you’ll find that in many instances our advice changes for each phase. We’re going to tell you a bit about each phase here—first from a Western and then from an Eastern perspective—so that you’ll have a basic understanding of the mechanics underlying the practical applications to come.
Phase 1: Menstruation
In the hours before you get your period, blood vessels in the endometrium lining the uterus tighten up, restricting blood flow to the endometrium. This causes it to start to die. Then the blood vessels relax again, which triggers the gradual shedding of the endometrium: you get your period. This goes on for between four and seven days on average, with the majority of the shedding happening in the first twenty-four hours, and so the heaviest bleeding occurs on the first or second day.The endometrium wastes no time in starting to regenerate itself so it will be ready to accommodate an embryo later in the cycle. Regrowth begins within two days of when you start to bleed. By day 3, estrogen and progesterone receptors form in the endometrium, and your hormones take control of endometrium building. By day 6, the endometrium is about 1 to 2 mm thick.
While all this is going on, the ripening process starts in the follicles on your ovaries that house your eggs, one egg to a follicle, beginning on about day 4. The follicles are less than 4 mm across at this point, but they will quintuple in size, to about 20 mm.
The whole shebang—endometrium shedding, endometrium regenerating, follicles growing—is orchestrated by the interplay of hormones. Estrogen and progesterone kick it off; their falling levels signal the release of gonadotropin-releasing hormone (GnRH), which triggers menstrual bleeding. GnRH in turn signals the release of follicle-stimulating hormone (FSH), which, appropriately enough, stimulates the growth of follicles.
Phase 2: Pre-ovulation
During menstruation, a signal from the pituitary gland at the base of the brain causes FSH levels to start to rise. The FSH stimulates the ovaries, causing fifteen to twenty follicles on them to begin ripening in preparation for release of the egg each one holds. In this phase, FSH continues to rise, still signaling to the follicles. Each follicle produces estrogen—the estrogen that will determine when you ovulate—so estrogen rises gradually through this phase.Full follicle development usually takes about two weeks, from day 4 or so of your period up to ovulation, though a normal follicular phase may last anywhere from ten to twenty days. Typically, by day 6 or 7 of your cycle, one follicle has become dominant, and the rest simply shrink away.
Rising estrogen tells your body to reduce FSH, so no more follicles are produced. Estrogen levels peak on about day 12 or 13, signaling the pituitary gland for the release of luteinizing hormone (LH), which in turn triggers the release of an egg. (In men’s bodies, sperm production also proceeds under the influence of FSH and LH.)
As estrogen levels rise, you may notice that your vagina feels more moist and your cervical mucus shifts from being sticky or rubbery to being opaque, whitish, and creamy, like hand lotion. This is not yet fertile cervical mucus. As estrogen rises even higher, it will thin out and become transparent and stretchy, like egg whites.
All the while, the endometrium has been regenerating. It thickens further as estrogen rises, and by ovulation a healthy endometrium will be about 7 to 10 mm thick.
Phase 3: Ovulation
Ovulation typically occurs on day 14 of an average cycle, although it could be as early as day 10 or as late as day 20, and as long as the cycle is regular, it’s nothing to worry about. It usually occurs within twenty-four hours of the surge in LH. Occasionally, a cycle produces no egg, which is normal. From time to time, two eggs are released, always within twenty-four hours of each other. This is how we get fraternal twins. Twenty-four hours after ovulation, progesterone levels rise to the point at which any further release of eggs is impossible. Contrary to popular belief, ovulation doesn’t necessarily alternate from one ovary to the other from month to month.Whichever side it comes from, the egg is about the size of the period at the end of this sentence. Upon its release from the lead follicle, the egg is quickly drawn into the fallopian tube, a process that takes only about twenty seconds.
The egg remains alive for twelve to twenty-four hours, traveling toward the uterus. When its time is up, if the egg has not been fertilized by the sperm, it disintegrates and is eventually reabsorbed into the body. If an egg is going to be fertilized, it happens within a matter of hours of ovulation, probably by a sperm that’s been waiting there for it. Sperm need fertile cervical mucus to swim through to reach the egg—mucus in which they can survive for up to three or four days—and that mucus is produced thanks to increasing estrogen levels in the later days of the follicular phase.
Within eighteen to twenty-four hours of ovulation, progesterone thickens the mucus, and it once again becomes opaque and no longer conducive to sperm survival or movement. Once an egg is fertilized, it continues on its way toward the uterus, a journey of about six days.
Phase 4: Potential Implantation
Progesterone is the dominant hormone in the second, post-ovulation half of your cycle, known as the luteal phase. Progesterone switches off production of FSH and LH, thereby preventing the release of any more eggs. It thickens the endometrium and helps it secrete nutrients so that it will be ready to nourish an embryo if called upon to do so. And it causes your basal body temperature (BBT) to rise (which is favorable to implantation), closes the cervix, and thickens cervical mucus, forming a plug that is meant to prevent more sperm from entering the cervix after fertilization. All this progesterone is released by the corpus luteum, which is formed from the collapse of the empty follicle that released the egg.
The length of the luteal phase is determined by how long the corpus luteum lasts, generally between twelve and sixteen days. The luteal phase needs to last at least eleven to twelve days, or there won’t be enough time for an embryo to implant, and even if an egg has been fertilized, no pregnancy will result or an early miscarriage may take place.
The embryo arrives in the uterus by the fifth or sixth day of the journey, cells busily dividing all the way. Generally it implants—attaches to the endometrium—within a day after that. The uterus itself assists with implantation by actually pressing its front and back walls together, holding the embryo in place. To achieve this, the body removes fluid from the endometrium in a process called pinocytosis.
Once the fertilized egg has nestled into the endometrium, it gives off the pregnancy hormone human chorionic gonadotropin (HCG), the signal your standard pregnancy test “pee stick” is looking for. That HCG also tells the corpus luteum back in the ovarian wall to continue releasing progesterone to sustain the endometrium, instead of shutting the uterine lining down after the usual twelve to sixteen days. (Five to six weeks later, the placenta will take over the task of producing progesterone and maintaining the endometrium.) At this point, the menstrual cycle is effectively finished (for the next nine months, anyway), and pregnancy has begun. After eight weeks of pregnancy, the little ball of cells has developed and differentiated enough to officially be known as a fetus.
If there is no fertilization or no implantation, the corpus luteum begins to degenerate, ultimately stopping the flow of progesterone, and the blood vessels leading to the endometrium begin closing down, in preparation for endometrial shedding. Estrogen levels begin to fall as well, and the drop in estrogen and progesterone trigger the release of GnRH and FSH, bringing the process full circle: the next period begins. The drop in progesterone is what may cause the symptoms we know as premenstrual syndrome (PMS).
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