The Menstrual Cycle
Female fertility is cyclical. During each menstrual cycle, there is a sequence of events influenced by hormones, resulting in fertile times and infertile times. The hormonal patterns within a cycle repeat throughout a woman's fertile life, or approximately forty years. The two ovarian hormones, estrogen and progesterone, dominate the reproductive cycle.
The length of a menstrual cycle is the interval between the first day of one menstrual period and the beginning of the next. It averages about four weeks. The menstrual cycle is divided into three phases: the pre-ovulatory phase, the ovulatory phase, and the post-ovulatory phase.
The Pre-ovulatory Phase The pre-ovulatory phase begins on the day the menstrual flow starts. It is variable in length.
What happens in this phase?
In this first phase, the woman’s body prepares itself for ovulation. In the ovaries, a few primitive ova start to develop. Throughout the growing process, the follicle surrounding the developing ovum produces a hormone called estrogen. As ovulation approaches, estrogen acts on the whole body system causing cyclical changes. Conditions change to become favourable to the survival and transportation of the deposited sperm: the cervical mucus becomes wet and more lubricated, the cervix becomes softer and more open, and the lining of the uterus (the endometrium) thickens to potentially receive a fertilized egg.
The length of this first phase varies. Ovulation occurs sooner or later depending on conditions within a woman’s lifestyle: stress, season, emotions, nutrition and health. It is the variation of this phase that determines the woman’s cycle length (see figure below).
Ovulation Ovulation occurs after estrogen reaches its peak level. The pituitary within the brain center secretes a hormone to ovulate. The follicle then bursts open and releases the ovum which is picked up by the fringes of the fallopian tube, and is guided into it. At ovulation, the presence or absence of sperm cells determines the fate of the released ovum. It will live for a maximum of 24 hours (averaging between 8 and 12 hours). Occasionally, a second ovulation may occur a few hours after the first one; however, both ova will disappear within the 24 hour ovulation window, if not fertilized. If the ova are fertilized by separate sperm cells, non-identical twins are conceived.
The Post-ovulatory Phase The post-ovulatory phase begins after ovulation and ends on the last day of the cycle (the day before menstruation begins again). It is constant in length.
After ovulation has occurred, the follicle transforms into the corpus luteum which releases a hormone called progesterone into the woman’s bloodstream. The body is then ready to protect the potentially fertilized egg. At this stage, the presence of progesterone in the body makes the cervix firmer and the cervical opening smaller. Cervical mucus thickens and forms a kind of gelatinous plug which protects the uterus from outside elements. Progesterone also causes the lining of the uterus to build up in preparation in the event of implantation of a developing baby.
For the next 12 to 16 days, two scenarios are possible:
- For one week, progesterone is released into the bloodstream. If fertilization and implantation have taken place, the production of progesterone increases, and plays a determinant role in sustaining pregnancy.
- Or, for one week, progesterone is released into the bloodstream. If there is no pregnancy, the release of progesterone slows as the corpus luteum degenerates, and then the lining of the uterus will shed during the next menstruation.
With the help of the Sympto-Thermal test, a couple can observe the effects of estrogen and progesterone in the woman’s body and thus identify the time of ovulation. Through the knowledge gained by the self-observation of external signs influenced by these hormones, a woman can understand her cycles. This is the basis of the Sympto-Thermal Method of Natural Family Planning.