The Fertility Cycle


The fertility cycle is the menstrual cycle viewed from the perspective of when in the cycle the woman is infertile or when she is potentially fertile.

How many phases in the Fertility Cycle – when do they occur?

There are three phase in the fertility cycle as shown on Fig. 6-13 below.

Fig 6-13 MucusThe three phases of the fertility cycle are labelled the pre-ovulatory infertile phase, the peri-ovulatory fertile phase, and the post-ovulatory infertile phase; see Fig. 6-14

Fig 6-14 Mucus

How long does the ovum live? How long does the sperm live? 

The ovum lives for 24 hours. The sperm can survive for up to 5 days in oestrogenic fertile-type mucus.

Do the phases of the menstrual cycle and the phases of the fertility cycle correspond with each other?

The phases of the menstrual cycle and fertility cycle do not correspond with each other; see Fig. 6-15 below for further details.

Fig 6-15 Mucus

When is the fertile phase in the cycle and how long does it last?

In the ‘symptothermal double-check method’ of natural family planning the fertile phase begins with the first sign of cervical mucus at the vulva, cross-checked with the ‘calendar rule’ whichever comes first, and it ends when there is a sustained rise in the basal body temperature (BBT) cross-checked with the cervical mucus symptom whichever comes last. The LIMITS of the fertile phase are defined by the RULES of the Natural Family Planning Method and the couple must learn these rules from a qualified Natural Family Planning teacher. The potential fertile phase of a couple is the time from the first act of intercourse which may lead to pregnancy to the demise of the ovum.1 According to Freundl 4 there are only 6 – 9 days of the menstrual cycle on which intercourse may result in pregnancy. The fertile phase normally occupies not more than one-third of the cycle.3

Does ovulation occur on the same day in every cycle and is it always in mid-cycle? 

The day of ovulation is not fixed in the middle of the cycle, but it varies from one cycle to another. It usually occurs about 14 days before the next period because the luteal phase after ovulation has a relatively constant length of 14 days, (range 10-16 days). [If the cycle length is 30 days, ovulation probably occurred on Day 16 of the cycle (30 minus 14 = 16), and if the cycle length is 25 days, ovulation would probably have occurred on Day 11 of the cycle, (25 minus 14 = 11)].

Does ovulation occur just once in the cycle? 

If two ova mature in any particular cycle, as in the case of twins, both ova will be released within the same 24 hour period.


  1. Wilcox AJ, Weinberg CR, Baird DD; ‘Timing of sexual intercourse in relation to ovulation – effects on the probability of conception, survival of the pregnancy, and sex of the baby'; N Engl J Med 1995; 333;1517-21.
  2. World Health Organization; ‘Temporal relationship between indices of the fertile period’,1983 page 1; Fertil & Steril; 39; 647-655. (Dr Anna Flynn was one of the WHO task force investigators for this study).
  3. Klaus, Hanna; ‘Natural family planning – Is it scientific? Is it effective?’ page 1; Newman Lecture Series 1; May 21, 2000. (on the internet if you google ‘natural family planning, Dr Hanna Klaus’.
  4. Freundl G et al; “Estimated maximum failure rates of cycle monitors using conception probabilities in the menstrual cycle”; Human Reproduction, (2003), vol 18, no12, p 2628-2633.
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.
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