A man and woman together are fertile for only six days during a women’s menstrual cycle. These 6 days include the day of ovulation and the 5 days before. The timing of these 6 days of fertility (also called the "fertile window") can change from cycle to cycle. The figure below illustrates the variability of the fertile window in 3 consecutive menstrual cycles. The fertile window is indicated by the blue color days, the red color days indicate the days of menstruation (or the woman’s period), and the green days are the days of infertility.
The first cycle in Figure 3 is 29 days in length, and the fertile window begins on day 10. The second cycle has 25 days, and the fertile window begins on day 6; the third cycle is 34 days in length, and the fertile window begins on day 17. The green infertile days before ovulation are absent in the second cycle since ovulation occurred early. In the third cycle the green days before the fertile phase are longer because ovulation occurred later in this cycle. This illustrates that the day of ovulation and the fertile window can vary from cycle to cycle. It also shows that the post-ovulatory phase is the more stable time of the menstrual cycle and that most of the variability occurs before ovulation. The post-ovulatory phase in the three cycles below ranges from 12-14 days, while the pre-ovulatory phase ranges from 11-23 days.
Fig 3: Variability of the menstrual cycle and the fertile window:
Natural family planning teaches you how to monitor your fertile phase and the variability of the fertile phase from cycle to cycle. Natural family planning utilizes natural markers of fertility to tell you the beginning, peak, and end of the fertile phase. Your fertile window will be longer than 6 days due to the variability of the natural markers.