Thisis based on the fact that a woman’s basal body temperature (the temperature after rest) rises in the days immediately after ovulation. The rise can be as much as 1° F (0.6°C); in some women it may be much less, for instance 0.4°F (0.2°C). Most women also exhibit a drop in the BBT just before this rise, which can help pinpoint it. The rise occurs because of the rise in the body’s progesterone level after ovulation.
Many women’s temperature changes are not immediately obvious — especially as the temperature tends to fluctuate somewhat during other parts of the month — so records should be kept for at least six months before the method is used. By doing this you can train yourself (or be trained) to interpret your own personal temperature patterns. The temperature should be taken ideally at approximately the same time each morning, before 7.30a.m., and before getting out of bed, doing any activity at all, or taking any hot food or drink. It must be taken after at least one hour’s complete rest; this means that if a woman has been up with fractious children in the night, she can still take an accurate temperature reading provided she has rested for at least an hour previously.
The temperature can be taken either orally, rectally or vaginally; rectally is most accurate. The temperature level is plotted in a special graph designed to show up small changes. The temperature rise needs to be sustained for twenty-four hours before it can be definitely attributed to ovulation, so two clear days should be left after the first rise before sex is safe — one day to ensure that ovulation has taken place, and one further day until the ovum’s life-span is over. The evening of the third day should be safe.
Advantages and disadvantages
Theis more accurate than the calendar method, since it is possible to pinpoint ovulation with some accuracy, but it has the same drawback that it cannot predict when ovulation will take place. As a result, sex has to be avoided for four days before the earliest possible time for ovulation — hence the need to keep records of previous cycles. Because of this uncertainty, the Which? Guide to Birth Control says: ‘The on its own works best if you restrict sex to the second half of the cycle.’
Not all women experience a very sharp and sudden temperature rise. Some women’s temperature patterns are step-like, zig-zag or ‘double-shifted’ (showing two peaks). Also, a woman doesn’t always experience the same pattern of temperature rise each month. To complicate matters, the temperature can be affected by childbirth, abortion or, activity before taking the temperature, high levels of in the blood, smoking a cigarette, physical or emotional upset, sleeplessness, , travel, and almost any illness — not just those producing high fevers. All of these factors can at times contribute to temperature charts that are confusing and difficult for the woman herself to interpret.
In addition, as Howard Shapiro says in The Birth Control book ‘the burden of charting temperatures in the early morning when you are only half awake may lead to inaccuracies both in taking the temperature and in recording it.’
However, even if you are as heavy-lidded as an iguana early in the morning, all is not lost. There is one school of thought which says that you can choose instead to take your temperature either at 5p.m. Or at bedtime each day, whichever fits best into your routine. If you take your temperature at 5p.m., it will consistently be approximately 0.7°F (0.39°C) higher than your morning BBT. If you take it at bedtime, it will only be approximately 0.3°F (0.17°C) higher. The temperature pattern over the month should echo that of your BBT.
This knowledge is also useful for occasional lapses in early morning temperature-taking. If things have gone wrong in the morning (for instance if you’ve overslept), you can take your temperature at 5p.m. And deduct 0.7°F (0.39°C) before plotting it on your chart or you can take it at bedtime and deduct 0.3°F (0.17°C). As a rule it is not a good idea to mix the three timings — if you choose this method, stick to the same time each day as much as possible.