What is the symptothermal method?

Bottom lineThe symptothermal method combines cervical mucus observations (to open the fertile window), basal body temperature (to close it, on the third high-temperature day after peak mucus), and a calendar backstop; in the largest study it had about 0.4 pregnancies per 100 women per year with abstinence on fertile days and about 1.8 in typical use, making it the most effective fertility awareness method, though it needs daily charting and 3 to 6 cycles to learn.

The symptothermal method is the combination form of fertility awareness: it uses cervical mucus to spot the fertile window opening, basal body temperature to confirm it has closed, and a calendar rule as a backstop. Because each sign covers the others' weaknesses, it is the most effective fertility awareness method for avoiding pregnancy.

The rules, in brief

• The fertile phase opens at the first sign of cervical mucus or on a calendar cut-off based on your shortest cycles, whichever comes first
• It closes on the evening of the third day of higher temperatures after the peak mucus day, when both signs agree
• On fertile days, couples avoid sex or use a barrier method

How effective is it

In the largest study, of 900 women using the Sensiplan version over more than 17,000 cycles, the pregnancy rate was 0.4 per 100 women per year with abstinence during the fertile phase and about 1.8 per 100 in typical use, comparable to many hormonal methods.

What it takes

Daily temperature on waking, daily mucus observation, a chart, and usually 3 to 6 cycles of learning. Most programmes recommend a trained instructor. Illness, alcohol, shift work, and postpartum or perimenopausal cycles make the signs harder to read.

See what is basal body temperature, how to track ovulation with cervical mucus, and the full beginner's guide to fertility awareness methods.

Femora keeps temperature, mucus, and cycle dates on one chart.