A brief outline of the Menstrual Cycle is on this page.
For further information on the menstrual cycle click on any topic listed below.
- What is the Menstrual cycle (6a)
- The Hypopituitary-Pituitary-Ovarian axis, (HPO Axis) (6b)
- The ovary and uterus during the menstrual cycle (6c)
- Factors necessary to achieve pregnancy (6e)
Brief outline of the Menstrual Cycle
What is the menstrual cycle?: The menstrual cycle is the fertility cycle of the woman during the child-bearing years.
When does the cycle begin and end?: The first day of the menstrual cycle is the first day of the menstrual period and the last day of the menstrual cycle is the day before the next menstrual period. The cycle length varies from 23 to 35 days, the average length is about 28 days.
What happens during each menstrual cycle?: During each cycle two events occur simultaneously:
- an ovum (egg) matures in the ovary and is then released into the fallopian tube ready to be fertilised. The release of a mature egg is called ovulation. (Link to Fig. 6.10 ovulation).
- at the same time that the ovum (egg) is maturing in the ovary, the lining of the uterus is being prepared to accept the fertilized ovum, i.e. the lining of the uterus becomes thicker and more vascular in preparation for a possible pregnancy.
What is menstruation? Menstruation is the vaginal bleeding that occurs at the end of an ovulatory cycle when fertilization has not occurred and is due to the drop in hormone levels in the bloodstream. Menstruation marks the beginning of a new cycle, and lasts 3 to 5 days.
Please explain the hormone bit: During the cycle the two female hormones, oestrogen and progesterone are low at the beginning of the cycle, their levels increase during ahe cycle and they are low again at the end of the cycle. During the cycle the lining of the uterus proliferates to prepare for a possible pregnancy due to the actions of two hormones, oestrogen before ovulation, and progesterone after ovulation. If pregnancy does not occur the levels of these hormones start to fall about 7 days after ovulation. As progesterone is required to maintain the integrity of the lining of the womb (endometrium), the drop in progesterone means that the endometrium can no longer be maintained and it is shed about 14 days after ovulation. This shedding is called menstruation. If fertilization of the ovum does not occur in that cycle menstruation will start about 14 days after ovulation. In other words when menstruation starts the woman knows that ovulation most likely had occurred 14 days previously.
What is ovulation ?: Ovulation means the release of a mature ovum (egg) from the ovary into the fallopian tube. It occurs once in any cycle, about 14 days before the next period. If two or more ova (eggs) are released in a cycle, all are released within the same 24 hour interval.
What is the life-span of the ovum?: The ovum lives for 24 hours.
What is the life-span of the sperm?: sperm to survive for up to 5 days in fertile-type cervical mucus present in the vagina during the 6 days or so leading up to ovulation.
Two questions about ovulation:
- Can the time of ovulation be pin-pointed in regular cycles ?
- Can the time of ovulation be pin-pointed when the cycle is irregular ?
Answer: The exact time that ovulation occurs cannot be pin-pointed by the fertility indicators, i.e. ’cervical mucus’ or by ’changes in the cervix itself’ whether the cycle is regular or irregular. Ovulation is confirmed by the ‘thermal shift’.
If you cannot pin-point the moment of ovulation, is this not a drawback for those who wish to achieve pregnancy and who may have irregular cycles?:
Answer: The answer is NO. If you do not know the exact moment of ovulation this is not a drawback to achieve pregnancy, as the life-span of sperm in fertile-type mucus is 5 days and fertile-type mucus is present in the vagina at the time around ovulation itself as well as the 6 days or so leading up to ovulation, i.e. the fertile phase of the cycle . This means that whether the woman has regular cycles or irregular cycles, she is still able to identify the fertile phase of the cycle by the presence of fertile-type cervical mucus at the vulva, the time when there is an increased probability of achieving pregnancy. The thermal shift is the only indicator that confirms that ovulation has occurred.
What factors can cause irregularity of the menstrual cycle? The menstrual cycle is regulated by a finely tuned balance of hormones secreted by the ovary and the pituitary gland, and is ultimately under the control of the higher centres of the brain. This delicate mechanism can be disturbed by a number of factors, notably stress, but also by over-doing physical exercise, by dieting and by weight-loss.
What effects has the ‘Pill’ on the menstrual cycle?: Women taking hormonal contraception (the Pill) do not have a normal menstrual cycle and the apparent menstrual period when taking the Pill is just a withdrawal bleed due to a drop in synthetic hormone levels in the blood in the pill-free days.
Menarche: The first menstrual period is known as menarche and usually occurs about 13 years of age. The normal age of menarche varies from 10 to 16 years. A study done in Ireland of 2940 girls aged 9 to 17 years showed that the average age of menarche in Ireland is 13.5 years, with a standard deviation of 1.3 years.1 The age of menarche has decreased in the 20th century and this could well be due to improved nutrition. In the 19th century girls in the USA reached a weight of 46kg at about 14 years of age which was the year of the menarche. In Belgium in the 19th century girls reached a weight of 46 kg at about 16.5 years and also attained the menarche at that age. According to Griffin et al,2 the increase in body fat at adolescence of about 11kg is equivalent to 99,000 calories and the number of calories estimated to maintain a pregnancy is 88,000. Thus one of the main functions of the adolescent growth spurt in females may be the storage of energy to sustain a pregnancy and lactation.
Menopause: When the final menstrual period has occurred the woman has reached menopause, and the end of the child-bearing years has arrived. Menopause occurs within a wide age range between the ages of 42 to 58,3 on average at about age 51 and this is unchanged from ancient times. (Link to Menopause)
1 Hoey, H.; The age of Menarche in Irish girls; Ir Med J. Oct. 1986, vol 79, no. 10, p283-285
2 Griffin, James E; Ojeda, Sergio R; Textbook of Endocrine Physiology, 3rd edition, 1996, ch. 9, p187, 188
3 Soules MR, Sherman S, Parrot E et al; Executive summary: Stages of Reproductive Aging Workshop (STRAW). Fertil Steril. 2001; 76: 874
To be most effective, the woman must be taught the Symptothermal Double-Check Method of Natural Family Planning by a qualified natural family planning teacher.
