How can the woman identify the fertile phase of the cycle?

By becoming familiar with the changes in her body associated with the fertile and infertile phases of the menstrual cycle, (fertility awareness), the woman will be able to identify when the fertile and the infertile phases of the cycle begin and end. These signs in her body which the woman can detect herself are called the fertility indicators, (‘cervical mucus’ and ‘basal body temperature, BBT)’, and are due to the effects of the ovarian steroid hormones oestrogen and progesterone.

The fertility indicators are classified as major indicators which can detect either the beginning or end of the fertile phase, and minor indicators which are inconstant and do not detect the beginning or the end of the fertile phase.2 The scientific basis for these effects are well described in the study by Moghissi.1

Major Indicators (signs of fertility):

Minor Indicators:

An example of a minor index is ovulation pain (mittleschmerz), or the ‘calendar calculation’.

A brief explanation of what the woman does when she uses the symptothermal double-check method of NFP:

In the symptothermal double-check method, the woman takes her temperature each morning and observes the presence or absence of cervical mucus at the opening of the vagina. This mucus comes from the glands lining the cervix i.e. the neck of the womb. The woman records  these two signs every day in a special chart, and also records the length of her previous cycles, (the calendar data). These recorded observations enable her and her spouse to determine, day by day during her cycle, whether that day is fertile or not. These observations become routine in a short while, are simple to learn, and women themselves are pleased to know when the fertile time begins and ends.


  1. Moghissi KS, Syner FN, Evans TN: ‘A composite picture of the menstrual cycle’; Am J Obstet Gynecol, 1972; 114:405- 418. Commenting on this study by Moghissi, Dr M.L. Taymor, Boston, Massachusetts states on page 416, “The special contribution of this paper is the correlation of the changes in the end-organ responses, i.e. BBT, and changes in cervical mucus, with the hormonal changes. As a result, these changes which are naturally of more interest to the clinician, are placed on a firmer physiologic basis.”
  2. Flynn A, Brooks M; ‘The Manual of Natural Family Planning’; 1996; ISBN 0 7225 3115 X