Fertility awareness methods and the fertile window
Fertility awareness methods estimate the fertile window (days when sex can lead to pregnancy) from cycle length, temperature on waking and cervical mucus (fluid from the neck of the womb).
Remember
Clear, stretchy cervical mucus warns that the fertile window is opening, whereas a sustained rise in basal body temperature (temperature on waking) only confirms afterwards that ovulation has happened. With typical use (real life, including mistakes and missed days), about 24 in 100 women relying on these methods become pregnant within a year.
Memory aid
mucus warns, temperature confirms
Cervical mucus gives warning that the fertile window is opening, while the temperature rise confirms afterwards that ovulation has happened.
A fertility awareness method avoids pregnancy by observation, and it does not change hormones. It does not stop ovulation, so the couple need to record and interpret the signs every day, then avoid unprotected sex or use a condom on the fertile days.
The first sign is cervical mucus. Rising oestrogen turns the thick, gummy mucus of the infertile days into clear, slippery, stretchy mucus that helps sperm swim through and keeps them alive. In fertile mucus sperm can survive for up to five days, so the fertile window opens before the egg is released. The window is approximately five days before to several hours after ovulation. Daily mucus records give early warning that ovulation is near.
The second sign is basal body temperature, the temperature taken on waking before any activity. After ovulation the corpus luteum, a temporary structure in the ovary that makes hormones, secretes progesterone. Progesterone is thermogenic, which means it raises body temperature, so the temperature rises slightly and stays raised for the rest of the luteal phase, which is the second half of the cycle. The rise comes only after the egg has gone, so it looks back and confirms that ovulation has happened. It gives no advance warning.
Typical use means real life, with mistakes and missed days, whereas perfect use means following the instructions exactly. Perfect use is 91% to 99% effective, but the typical-use failure rate (the proportion of women who become pregnant in a year) is about 24% a year, or roughly one couple in four. That failure rate is higher than that of the male condom (about 18%), though better than using no method at all (about 85%). These methods can take two to three cycles to learn and may not suit women with irregular periods.
Key facts
- Fertile window
- approximately five days before to several hours after ovulation
- Early warning
- clear, slippery, stretchy cervical mucus appears in the days before ovulation, driven by rising oestrogen
- Confirmation
- a sustained rise in temperature on waking, caused by progesterone, shows that ovulation has already happened and does not predict it
- Cycle length
- counted from the first day of bleeding, it is one input to these methods, but it is a count of days, not a body sign
- Typical use
- about 24 in 100 women become pregnant in a year, roughly one couple in four
- Perfect use
- 91% to 99% effective when the instructions are followed exactly
- Learning time and who it suits
- it can take two to three cycles to learn, and it may not suit women with irregular periods
Exam traps
- A surge of luteinising hormone (the burst of hormone that triggers release of the egg) is not itself the cause of the sustained rise in basal body temperature, because that rise follows ovulation and is attributed to progesterone from the corpus luteum.
- Thick, gummy cervical mucus does not warn that the fertile window is opening, because it is the pattern of the infertile days and returns after ovulation, when progesterone takes over.
- A sustained rise in basal body temperature does not warn in advance that the fertile window is opening, because it follows ovulation and confirms that the fertile period is ending.
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