The starting point: the fertile window
Every fertility-awareness method rests on one biological fact. A cycle contains only about six fertile days: the five days before ovulation and the day of ovulation itself. Two durations explain that figure:
- Sperm survive up to five days in the cervix — but only if they find fertile mucus there. Without it they die within hours.
- The egg can only be fertilised for 12 to 24 hours after ovulation. After that the cycle is over as far as fertility goes.
The difficulty lies in a second fact: ovulation does not happen on a fixed day. The second half of the cycle (the luteal phase) runs fairly consistently at 12 to 16 days, but the first half varies a great deal — from cycle to cycle in the same woman, under the effect of stress, travel, illness or jet lag. That is why any calculation based on the calendar alone is structurally fragile: it predicts an event that refuses to be predictable. The symptothermal method does not predict — it observes.
Two signs, two different jobs
| Sign | What it shows | When |
|---|---|---|
| Cervical mucus | That the fertile window is opening. Under rising oestrogen it becomes abundant, clear and stretchy — and makes sperm survival possible. | A sign of warning: it speaks before ovulation |
| Waking temperature | That the fertile window has closed. After ovulation, progesterone raises basal body temperature by 0.2 to 0.5 °C, and it stays up until the next period. | A sign of confirmation: it only proves ovulation afterwards |
Neither is enough on its own. Mucus opens the window without saying when it shuts; temperature shuts it without having given notice. The method is the crossing of the two, and that crossing is what explains the gap in effectiveness between this method and single-sign ones.
The closing rule
The post-ovulatory infertile phase begins once both conditions are met, taking whichever comes later:
- Temperature rule. Three consecutive readings higher than the previous six, the third being at least 0.2 °C above the highest of those six. The infertile phase opens on the evening of the third day.
- Mucus rule. Identify the peak day: the last day of stretchy, clear mucus or of the slippery, lubricated sensation. The infertile phase opens on the evening of the third day after that peak.
The rules for the start of the cycle — the days before mucus appears — are trickier and differ between teaching schools. That is precisely where a trained instructor makes the difference, and where self-taught errors concentrate.
Effectiveness
| Pregnancies per 100 women per year | |
|---|---|
| Perfect use (rules applied without deviation) | around 0.4 |
| Typical use (in a trained, supported population) | around 2 |
| Typical use (broader estimates, all practices) | from 2 to over 10 |
The most favourable figure comes from a large prospective German study of trained and supported users. It does not transfer mechanically to someone learning alone online: the gap between 0.4 and 10 is not about the method, it is about training and consistency.
What it actually asks of you
- Training. Allow three to six supported learning cycles before relying on it as contraception.
- A daily temperature, on waking, before getting up, at the same site each time (oral, vaginal or rectal — but always the same), after at least three hours of unbroken sleep.
- A daily mucus observation, recorded in the evening for the day just past.
- A decision as a couple: abstinence or condoms through the fertile window, on average seventeen days a cycle. That is settled before starting, not during.
When it does not hold
The method assumes a readable ovulatory cycle. It becomes unreliable, or unusable, after childbirth, while breastfeeding, approaching menopause, with polycystic ovary syndrome, on night shifts or very irregular hours, and for several months after stopping hormonal contraception while cycles find their own rhythm again. It gives no protection against sexually transmitted infections.