Cycle knowledge · 5 min read
How to identify your fertile days: calculating your fertile window
Would you like to calculate the fertile days in your cycle, or work out when your next period will arrive? Many cycle apps promise to do exactly that for you. Most of them, however, are based on the calendar method, which works with past values and averages. Their predictions therefore often do not match reality. In this article you will find out why your cycle is so difficult to calculate, and what alternative there is to determine your fertile days and the timing of your next period after all.
The female cycle
The cycle starts on the first day of your period. Your period begins on the day you first observe fresh, red blood. The bleeding is part of the first phase of the cycle, known as the follicular phase (or egg maturation phase). During this phase, 10-20 follicles mature in the ovaries. The aim of the cycle is for one follicle to release its egg during ovulation. This means an egg is released from the ovary into the fallopian tube, where it can be fertilised.
After ovulation, the second phase of the cycle begins: the luteal phase. During the luteal phase, the uterine lining that has built up is maintained so that a possible pregnancy can be carried. If no pregnancy has occurred, the lining is not maintained and bleeding occurs around 10-16 days after ovulation - your next period.
Cycle length
The length of a cycle is determined by the length of the egg maturation phase and the timing of ovulation. The phase after ovulation is very constant, at 10-16 days. If a cycle is longer than usual, it means that ovulation simply took place later. So it is not your period that is late, but your ovulation. Medically, cycle lengths between 23 and 36 days are considered normal. Variations of 7-10 days within a year are not unusual.
Cycle calendars
If you use a cycle calendar or a period calendar, for example in the form of an app, you enter the days of your period there. Cycle calendars like these “calculate” your cycle based on what is known as the calendar method.
Calculating your cycle with the calendar method
The calendar method uses the average length of your past cycles to “predict” how long your current cycle will be and when your next period will arrive. To calculate the fertile window, it is assumed that the cycle is 28 days long and that ovulation takes place on cycle day 15. The fertile window is calculated with the calendar method as follows: your shortest cycle minus 9 days marks the start of the fertile window, and your longest cycle minus 11 days marks its end.
Criticism of the calendar method
The calendar method is based on the assumption that a cycle is 28 days long and that ovulation takes place on day 15. Studies show that only 13.1% of cycles are 28 days long, and that ovulation occurs on cycle days 14-15 in only 25.5% of cycles.1 For the majority of menstruating people, these calculations are therefore unsuitable. And because the calendar method calculates on the basis of past cycles, it has no connection to what is currently happening in your body.
The egg maturation phase is the variable phase of the cycle. Ovulation can shift both earlier and later, and therefore cannot be predicted scientifically. The calendar method cannot tell you whether or when ovulation has taken place. What the calendar method calculates is therefore not well suited either for contraception or for trying to conceive.
Calculating your cycle with NFP
With NFP (natural family planning), you observe the cyclical changes in two body signs: cervical mucus and basal body temperature. Cervical mucus responds to the rise in the hormone oestrogen before ovulation and is therefore a good indicator of the beginning and peak of the fertile window. Only when ovulation takes place does the basal body temperature rise afterwards by around 0.2-0.5 degrees.
Such a rise in basal body temperature sustained over several days is the confirmation that ovulation has taken place. While the exact timing of ovulation cannot be reliably predicted from these body signs either, the fertile window can be narrowed down and confirmed retrospectively.
Once ovulation has been confirmed with NFP through the rise in basal body temperature, the timing of your next period can be calculated. The phase after ovulation is the stable phase of the cycle. It lasts between 10 and 16 days and is roughly the same length in each of your cycles. If your luteal phase is usually around 12-13 days, you can use your temperature rise to work out when your next period will arrive. The start of your period is often also signalled by a drop in basal body temperature in the morning.
The advantage of NFP
With NFP, you observe what is happening in your body every day. This means your observations and calculations reflect much more closely what is actually going on in your cycle. You can use NFP to observe your cycle, to conceive at the right time, or as a reliable, hormone-free method of contraception.
Conclusion
- You can only calculate your cycle to a limited extent. The calendar method works as a rough guideline, but only for those who have a healthy cycle with ovulation and a cycle length between 26 and 30 days. The calendar method does not show what is actually happening in your body.
- The calendar method is not suitable for contraception, and often not helpful when trying to conceive either.
- Predictions of ovulation are unreliable, as the egg maturation phase is highly individual and variable.
- By observing your cervical mucus, you can follow your cycle “live” and thus determine your real fertile days.
- By measuring your basal body temperature, you can confirm your ovulation retrospectively and use this to calculate the timing of your next period.
- NFP can be used both for contraception and when trying to conceive.
Source: 1 MFM Deutschland e.V. (2022) Zyklus und Fruchtbarkeit. Wissen und Fortbildungen. Retrieved on 02.09.2022 from https://www.mfm-programm.de/index.php/wissens-blog/zyklus-und-fruchtbarkeit
About the author
Katharina Dinzen is an expert in natural family planning (NFP) and cycle health. With Ovulista, she shows women and girls how to use their cycle for their own wellbeing, for trying to conceive, or for hormone-free contraception.
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