Fertility Education Trust

guidelines to charting your fertility cycle, page 24 of 26

4. The Double-Check Concept: The Calendar / Mucus Double-Check Rule (before ovulation)

If you have no diary records of your previous cycle lengths:

If you have no records of natural cycles, then you really have nothing to guide you. Therefore, you should start by using the post ovulation infertile time only for a while. Gradually you can include the infertile days before ovulation as you learn to understand your mucus pattern and can see how long your cycles generally are.

Cycles 1-3	Do not use first phase at all till you know your cycle better and can identify mucus accurately. Use the Post Ovulation Phase only.
Cycles 4-6	Provided none of the previous three cycles was less than 26 days, the first five days of cycle, which include the period, are infertile (provided they are DRY with no mucus). If any cycle was less than 26 days, use the next rule immediately: Shortest cycle - 21 days, which will reduce the infertile time to less than five days. In very rare cases, women with very short cycles have conceived from intercourse on Day 5.
Cycles 7-12	Use the stricter Rule: Shortest cycle minus 21 = last infertile day, if still dry.
Cycles 13+	Use the Standard Rule of shortest cycle minus 20 = last infertile day, if still dry.

If mucus appears before the day of the Calendar Rule, fertility begins immediately.