Male physiology

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.

  Physiology of the female fertility cycle

The female cycle is usually referred to as the menstrual cycle, menstruation being the most prominent event, however for our purposes, we will use the term fertility cycle, placing the emphasis on the cyclic changes of fertility.

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.

  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:-

  • The temperature remains on the lower level   When the oestrogens reach a certain level in the blood, the pituitary gland is stimulated to produce a sudden surge of LH (luteinising hormone) which precipitates ovulation within 36 hours. The most mature follicle ruptures and releases the ovum. - Ovulation


    Changes During the Fertility Cycle

      Post-ovulatory phase - controlled by progesterone
    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 endometrium softens in preparation for the implantation of a fertilised ovum

    • The cervix becomes lower, firmer and closed

    • Cervical mucus becomes hostile preventing sperm penetration.

      • After ovulation there is a rapid reversion to the infertile state - A dense network of filaments forms a thick sticky mucus plug which impedes sperm penetration. Sperm are rapidly destroyed by the acidic vaginal secretions
    • The temperature is raised by around 0.2C or more.
    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.


    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

    • The waking temperature
    • Cervical mucus changes
    • Changes in the cervix
    • Calculation of cycle length
    • Minor indicators of fertility for example abdominal pains and breast symptoms
    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'

    First indicator - Temperature
    Women are taught how to record, chart and interpret an accurate basal body or waking temperature, using specially-designed fertility charts and thermometers, preferably digital thermometers. The temperature rise or shift confirms ovulation. A sustained rise in temperature for 3 days indicates 48 hours past ovulation and determines the onset of the post-ovulatory infertile phase.

    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'

    Second indicator - Cervical mucus changes
    Cervical mucus changes are observed at the vulva (vaginal entrance) and are recorded at the end of each day. Mucus changes can also be observed at the cervix where it may appear one day before its appearance at the vulva. Developing awareness of cervical mucus changes takes time and persistence. These changes may be masked by seminal fluid, spermicide or vaginal infections, so it is vital that a woman receives adequate teaching and support to monitor this sign.

    • Dryness indicates the infertile phase

    • Sensation of moistness or dampness indicates potential fertility

    • Sensation of wetness, slipperiness and the presence of transparent, slippery, stretchy mucus indicates maximum fertility. This is the most valuable indicator for any woman experiencing difficulty conceiving.

    • Peak day is the last day of highly fertile-type mucus recognised retrospectively. (coincides closely with ovulation).

    • Following peak day there is a rapid return to dryness or moistness until the next menstruation.

    Some NFP organisations teach cervical mucus changes as a single indicator method. This is known variously as the Ovulation method, Billings Ovulation Method or the Creighton Model (Hilgers Method)

    More - Cervical Mucus - an Indicator of Fertility'

    Third indicator - Changes in the cervix

    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.

    • A low, long, tilted, firm, closed, dry cervix indicates infertility.

    • A high, short, straight, soft, open, wet cervix indicates fertility.

    More - Cervix Changes - an Indicator of Fertility'

    Recording cycle length

    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.

    A calculation can be made by taking the shortest cycle over the previous year and subtracting 20 to determine the last infertile day of the pre-ovulatory infertile phase.

    More - Calender Calculation

      Minor indicators of fertility
    • Ovulation or Mittelschmerz pain - A sharp pain or dull ache on one side of the lower abdomen for up to a few hours.

    • Breast symptoms - A characteristic tenderness or tingling sensation may be experienced around ovulation.

    These are the least reliable indicators but may be useful for some women to confirm other observations.

    See - A sympto-thermal chart showing correlation between indicators of fertilty.

    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.

    This page has outlined the physiology and introduced the indicators of fertility, this knowledge can be used for

    Planning Pregnancy.

    Avoiding Pregnancy.

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