How cervical mucus identifies the potential Fertile Days


The woman observes the presence of mucus at the vulva (i.e. the opening of the vagina).


Any observation of mucus at the vulva either by sensation or appearance is significant as it indicates potential fertility, and is a sign of ovarian activity and pending ovulation, (i.e. oestrogen is being secreted in the ovary by a developing ovum). In an ovulatory cycle the changing behaviour of mucus from when it is first observed at the vulva to its ‘peak’ is called the mucus ‘buildup’.2,6 The cervical mucus secretion is present for the six days or so leading up to ovulation.

BEFORE OVULATION – Effect of Oestrogen:

The Changing Pattern of the Mucus ‘Build-up’ to ‘Peak Day’ and Ovulation

  • 1. Secretion of less-fertile mucus:
    After the menstrual period there may be a sensation of dryness at the vulva for a few days. This is followed by the beginning of the secretion of mucus by glands in the cervical crypts due to oestrogen stimulation from the developing follicle in the ovary. The woman first observes mucus as a sensationof moistness or dampness at the vulva, and/or by a vaginal mucus discharge which has a thick, opaque and jelly-like appearance and which breaks on stretching between the fingers (tacky*). These descriptions are in keeping with less fertile mucus. The presence of the mucus symptom indicates the beginning of the fertile phase, as it indicates ovarian activity. (*Tacky means that the mucus breaks on stretching, and never stretches beyond 0.5cm.6)
  • 2. Secretion of more-fertile mucus:
    Then two or three days after the beginning of the mucus symptom the sensation at the vulva changes to one of wetness, slipperiness or lubrication, and/or the appearance of the cervical mucus is described as clear, like raw egg-white, and can hold a stretch, (‘spinnbarkeit’**)3. These observations are in keeping with more-fertile mucus. The wet, slippery sensation is of longer duration in young women and in parous women, and is of shorter duration in women approaching the menopause and in women who have taken hormonal contraception for several years.1 (** Spinnbarkeit means the mucus can hold a stretch which can be up to 10 to 20 cms with ‘good’ spinnbarkheit.3)

Peak Day:

  • 3. Peak Day is defined as the last day that mucus with more-fertile characteristics – slippery, stretchy, clear – appears at the vulva, irrespective of quantity.1 It can only be diagnosed the following day, when both the sensation and appearance have definitely changed to less-fertile mucus or to no mucus. It is the loss of slipperiness or wetness that helps define ‘peak day’. Peak day is not necessarily the day of maximum stretch and the quantity of mucus is usually not at its maximum on the Peak Day.1 Peak Day coincides with the day of ovulation in 80% of cases, and the probability of conceiving is highest on that day, i.e. Peak Day is the day of maximum fertility.1,2,4

AFTER OVULATION – the effect of progesterone on the mucus symptom:

In the ‘symptothermal double-check method’ of natural family planning the beginning of the fertile phase is identified by the presence of the Mucus Symptom, either by sensationand/or by the appearance of mucus at the vulva, cross-checked with the Calendar Rule whichever come first. The end of the fertile phase is identified by cross-checking the Temperature (BBT) Indicator with the Mucus Symptom whichever comes last.4 Sexual intercourse or genital contact during the fertile phase of the cycle may lead to pregnancy.6 The limits of the fertile phase are defined by the rules of the chosen natural family planning method applied to the fertility indicators (fertility awareness), and the woman must be taught these rules by a qualified natural family planning teacher.


  1. Odeblad Erik; ‘The discovery of different types of cervical mucus and the Billings Ovulation Method’ page 30; Bulletin of the Ovulation Method Research and Reference Centre of Australia, Vol 21, No3; 3-35; Sept 1994. (internet if you google ‘Erik Odeblad, cervical mucus’).
  2. Klaus, Hanna; bNatural family planning – Is it scientific? Is it effective?’ for ‘peak day’ see pages 6, 5, 4, 10; Newman Lecture Series 1;May 21, 2000. ( internet, google ‘natural family planning, Dr Hanna Klaus’).
  3. Cohen M, Stein I, Kaye B; ‘Spinnbarkeit: A characteristic of cervical mucus; significance at ovulation time’; Fertil & Steril; Vol 3, no 3, 1952.
  4. Flynn AM, Lynch SS; ‘Cervical mucus and identification of the fertile phase of the menstrual cycle’; Br J of Obstet &Gynec; August 1976; vol 83; 656-659
  5. Flynn AM, Brooks M; The Manual of Natural Family Planning, 1996, ISBN 0 7225 3115 X
  6. Klaus, H ; Natural Family Planning : A Review 2nd Edition. July 1995, pages 11, 13; NFP Center of Washington, D.C. Inc. 8514 Bradmoor Drive, Bethesda, MD 20817-3810

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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