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 endometrium (lining if the uterus) becomes thicker
• The cervix becomes higher and softer and open
• Cervical mucus produced by the glands or crypts in the cervix changes to a very ‘sperm-friendly mucus‘:
• Increased salts,sugar and amino-acids - to nourish sperm
• Increased fluid (to keep cells isotonic). Up to 10x increase in mucus volume
• Highly-fertile mucus is 98% water - Transparent, glistening, slippery, stretchy - spinnbarkeit effect.
• The structure of fertile mucus using nuclear magnetic resonance shows a loose network aiding sperm penetration.

• 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 0.2°C 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.
  

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  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.
  

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  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.