ThermometerBasal body temperature is one of the primary signs of ovulation and fertility which can be used as an ovulation calculator and in the production of a fertility chart. It is an easy method to help you get to learn your cycle better and find your fertile window, thereby allowing you to time sex more accurately and improve your chances of conceiving.


What is Basal Body Temperature?

Basal body temperature is defined as the lowest body temperature over a period of 24 hours. This usually occurs at night-time whilst you are sleeping.


How Does Basal Body Temperature Change During Your Cycle?

After ovulation has occurred the blood levels of the hormone progesterone rise. This has the effect of increasing your body temperature. This is usually only by about 0.4 to 1 degree Centigrade (0.7 to 1.8 degrees Fahrenheit) and so is not readily obvious. The temperature falls again as the progesterone levels fall at the beginning of your period in your next cycle.


How Can I Measure Basal Body Temperature?

The best way to measure your basal body temperature is by using a basal body thermometer or basal thermometer. These are different to the normal home thermometers as they are designed for women to use when trying to conceive. They are digital thermometers which are very accurate, usually to about one tenth of a degree, although some claim to be accurate to one hundredth of a degree. This ability to pick up small variations in temperature is essential in allowing you to monitor the small changes that occur in body temperature during your cycle. To learn more about the different types of thermometers available, check out the section: Basal Thermometers

Because the changes that you are looking for are so small, it is important to be as consistent as possible when measuring your temperature, so that other variables are not confusing the results. This means that you should record the readings at around the same time each day, and this should be done first thing on waking up in the morning after at least 3 hours sleep. The temperature reading should be the first thing that you do, before eating, drinking, getting up, or even talking!


Where Can I Get a Basal Body Thermometer?

These thermometers are readily available at chemists or online. Ensure that you are buying a thermometer for the purposes of fertility charting to ensure that it is able to accurately pinpoint the small changes in temperature that you are looking for.


What May Make Basal Body Temperature Readings Inaccurate?

There are various things which can affect your body temperature and thus make any readings inaccurate. These include:

  • Illness, especially if there is an associated fever, but even if this is not the case.
  • Some medications may also have this effect, including the use of progestogens. If this might be the case for you, I would recommend discussing your wish to produce a fertility chart with your doctor so that they can advise you if your medication would affect this. If your medication may impact upon your basal body temperature, you can still do fertility charting, but use the other fertility signs such as cervical mucus and positioning, rather than temperature monitoring. NEVER stop taking prescription medication without first discussing it with your doctor!
  • Drinking alcohol the evening before can cause your body temperature to rise, thus making the morning readings possibly inaccurate.
  • The surrounding environment also has an impact upon your body temperature, so if there is a change in the normal room temperature, or you are using an electric blanket, or cuddling up more to your partner, this will also make your readings fluctuate abnormally.
  • Your body temperature will also change on waking up and movements, so if you have had a disturbed or restless night, or needed to get up to go to the toilet within 3 hours of the time of your usual temperature measurement, then the values may also be affected.

If these situations occur it is still worth measuring your temperature as you would usually do. However, when recording on your fertility chart, it is advisable to either state these disturbances, or mark the reading as questionable so that you are not placing too much importance upon that reading.


How Can I Use the Results to Predict Ovulation?

When charting your basal body temperature, you are looking for a sustained rise in order to determine when ovulation has occurred. You should record the readings on a daily basis on your fertility chart, along with any variables that may have made the reading inaccurate.

It is not uncommon for you to have fluctuations in your basal body temperature. You may get an occasional “spike” in your temperature, but unless this is sustained for at least 2 – 3 days, it is unlikely to be an indicator that ovulation has occurred.

The basal body temperature rises at the same time as the increase in progesterone that occurs after ovulation. It cannot be used as a predictor of ovulation and therefore when to time sex for that cycle. By the time of the temperature change, ovulation has already occurred and the egg will have been lost as it only survives in the reproductive tract for up to 24 hours.

The idea of using basal body temperature to predict ovulation and thus fertility, is to give you a better understanding of your own cycle. By charting your temperature (along with other fertility signs) over the course of a couple of cycles, you will be able to produce an ovulation calendar which shows when ovulation is happening for you, and thus be able to time sex better for future cycles.


Can My Basal Body Temperature Predict Pregnancy?

As the levels of progesterone remain high during the early stages of pregnancy, the basal body temperature also remains elevated. You would normally expect the temperature rise to fall back to pre-ovulation levels around the time of your period being due, so if this doesn’t happen, it may be an indicator of an early pregnancy. However, if your period is late it is advisable to do a pregnancy test, so don’t just rely on a maintained temperature rise as an indicator.


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