Cycles vary in length from 23 days or less in a short cycle, to over 35 days in a long cycle. Few
women have an absolutely regular menstrual cycle, and a variation of up to 7 days is perfectly normal.
For convenience, we will use an average length cycle of 28 days.
Post-ovulatory phase - controlled by progesterone
A woman learns to monitor her fertility cycle subjectively
by observing physiological changes in her body,
using a combination of indicators of fertility.
These indicators are scientifically proven to reflect
changes in the ovarian hormone levels and to reflect
fertility status accurately.
Indicators of fertility
Fact file on sperm
A man is always potentially fertile, whereas a woman‘s fertility recurs on a cyclical basis.
For this reason we have to look in more detail at the woman‘s physiology to understand the phases
of fertility and infertility.
Sperm remaining in the vagina are destroyed within hours by the
acidity of the vaginal secretions.
For this reason genital contact or withdrawal method (‘being
careful‘) could cause pregnancy if fertile
mucus is present at the vaginal entrance.
Physiology of the female fertility cycle
Hormonal changes in the fertility cycle
The fertility cycle is under the control of sex hormones. It may be conveniently divided into two
phases - the phase before ovulation (pre-ovulatory) and the phase after ovulation (post-ovulatory)
Pre-ovulatory phase - controlled by FSH and oestrogen
The pituitary gland at the base of the brain secretes FSH (follicle-stimulating hormone) which,
stimulates the ripening of follicles in the ovary. The ripening follicles produce increasing
amounts of oestrogen.
As the oestrogen levels rise approaching ovulation, certain changes take place:-
Changes During the Fertility Cycle
Following ovulation, luteinising hormone or LH causes the ruptured follicle to develop into the corpus luteum,
the flower-like structure in the ovary which produces the second ovarian hormone - progesterone.
Under the influence of progesterone, the following
changes occur :-
The corpus luteum remains for around fourteen days,
then it shrivels and dies; the level of progesterone falls;
the temperature drops; and the endometrium disintegrates,
so completing the cycle.
The cyclic phases of fertility and infertility
This figure illustrates an average fertility cycle of 28
days. The first day of menstruation is day 1 of the cycle.
Subsequent days are numbered up to but not including the
first day of the next menstrual period. A number of
infertile days follow menstruation - this is the pre-ovulatory
relatively infertile phase. The fertile phase occurs
either side of ovulation. The first sign of cervical mucus
designates the onset of the fertile phase, because sperm
can survive in fertile mucus awaiting ovulation. After
ovulation, time must be allowed for ovum survival and the
possibility of a second ovulation occurring within 24 hours.
The post-ovulatory infertile phase is confirmed by a
combination of temperature and mucus signs about three
days after ovulation. This phase lasts until the onset of
the next menstrual period. The post-ovulatory infertile
phase is the most effective in avoiding pregnancy.
Variations in cycle length
The post-ovulatory phase or interval between ovulation
and the next menstrual period remains fairly constant -
around 14 days. As cycles vary greatly in length, it
follows that the interval between menstruation and ovulation
(pre-ovulatory phase) must constitute the variable
length of the cycle.
In a short cycle of 21 days, ovulation will occur around day 7
and there will be no pre-ovulatory infertile days. A
normal length cycle (around 28 days) will have a few
pre-ovulatory relatively infertile days and a long cycle
(for example 35 days) where ovulation does not occur until
around day 21, will have many pre-ovulatory relatively
infertile days.
The most effective method is a multiple indicator approach
generally the sympto-thermal method combining temperature
recordings with cervical mucus symptom.
More - Indicators of Fertility'
Temperature readings give no indication of the onset of the fertile phase and the temperature is of no value in timing intercourse to achieve pregnancy.
More - Temperature - an Indicator of Fertility'
More - Cervical Mucus - an Indicator of Fertility'
Detecting cervical changes can give additional information and is particularly useful for women with very long cycles, during breast-feeding or pre-menopausally. The first change in the cervix is frequently noted one or two days prior to changes in cervical mucus, and can give a very early warning of approaching fertility. It generally takes two or three cycles for women to be able to accurately detect these subtle changes in the level, position, consistency and the opening of the cervix.
More - Cervix Changes - an Indicator of Fertility'
The cycle length should be measured from the first day of a period (fresh red bleed) up to, but not
including the first day of the next period. Any spotting prior to a period should be included in the
previous cycle (pre-menstrual). This helps to determine the length of the pre and post-ovulatory phases
more accurately.
More - Calender Calculation
See - A sympto-thermal chart showing correlation between indicators of
fertilty.
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It is beyond the scope of this web site to give in-depth details on recording and interpreting fertility charts. Full details including anatomical illustrations and examples of normal and abnormal charts are included in the book and video 'Fertility'. These resources should be used with the support of a trained NFP teacher.
See - Education for Book, Video and Teaching.
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