How to chart your ovulation – Part 5

How to use Basal Body Temperature (BBT) to detect ovulation.

After a woman ovulates the empty egg sac (the corpus luteum) on the ovary produces progesterone which is a body warming hormone. We have already discussed the function of progesterone in preparing our body for a pregnancy, so now I will discuss how we can use it’s ‘’warming’’ property to detect ovulation.

The increase in progesterone which occurs after ovulation causes our resting body temperature (BBT) to rise in relation to what it was before ovulation. The temperature rise is only slight (0.2-0.4 degrees Celsius) but by plotting your morning temperatures on a graph daily throughout your cycle, you will be able to detect this rise. Once we see 3 days of higher temperatures, we can infer that ovulation has taken place the day BEFORE the temperature rise.  Here is an example of one of my own charts. Ovulation is shown by the vertical red line. The horizontal red line is my “coverline” – which represents my highest pre-ovulation temperatures and my lowest post ovulation temperatures.

The lower body temperatures are seen pre-ovulation and the higher body temperatures are seen following ovulation.


What is basal body temperature (BBT)?

Your basal body temperature (BBT for short), is your body temperature at rest and is measured first thing in the morning following at least 3-4 hours of solid sleep. You need to record your BBT before you get out of bed or do any activity using a digital ovulation thermometer. You can buy your ovulation thermometer here. Before a woman ovulates i.e. (during the follicular phase) her BBT is low (usually around 36-2 – 36.5 degrees Celsius. Following ovulation her BBT rises slightly (by half a degree). This rise can be detected and the difference between these lower and higher temperatures can be seen clearly when plotted on a graph. We call these graphs ‘fertility charts’ and we can infer that ovulation has occurred on the last day of the lower body temperatures.

Why is charting so helpful?

By observing your own fertile signs e.g. cervical fluid, on the lead up to ovulation and by doing ovulation tests it will help time intercourse correctly and you will hopefully have a fresh supply of sperm waiting in the fallopian tubes for the released egg. As the fertile window can last 4-6 days however it is also very useful to know when your fertile days are over so you know when your period is due. By charting your BBT can you pin point the day you have ovulated. Charting is also important if a woman has been trying to conceive for a while so she can predict on which day approximately, she ovulates each cycle. Of course, a woman can ovulate on different days each cycle so charting can also help woman with irregular and longer cycles know when ovulation has taken place. Most fertility issues faced by couples come down to bad timing and most woman can avoid unnecessary and often invasive tests if she knows her own pattern. Remember a majority of woman do not have a perfect 28 day cycle and do not ovulate on cycle day 14 each cycle. I for example  for the past 6 months have had a 28-34 day cycle and ovulate on cycle day 16-22. A woman’s cycles can change following giving birth as well, so it cannot be assumed that a if you have a 28 day cycle before having a baby you will return to this pattern after you have given birth.

Charting is also very useful in uncovering fertility issues such as annovulation (when a woman has periods but is not ovulating) – see more on this here.  Below is an example of a chart showing annovulation. There is no clear temperature shift confirming ovulation and I came on my period on cycle day 23.


When charting you BBT you can confirm ovulation has occurred following 3 day of higher temperatures and be sure that your fertile days are over. At this stage in your cycle you will say you are 3 days post ovulation (3dpo) and your other fertile signs e.g. fertile cervical mucus, should have subsided. If you see 3 high high temperatures and are still seeing egg white cervical mucus then there may be another reason for your higher temperatures and you should assume you may be still fertile. See here for more information that can cause higher body temperatures before ovulation (the main ones being disturbed sleep, taking your temperatures too late, having a hangover or being ill).

Once ovulation has occurred you BBT will remain above your pre ovulation temperatures for 12 to 16 days. A woman can also assume her period is due 12 to 16 days from the day of ovulation (depending on the length of her luteal phase). If you get your period sooner than 12dpo or see spotting then please see my other sections on a luteal phase defect and reasons for spotting before your period. If your temperatures stay high past the day your period is due or for 16 days it is time to do a pregnancy test. If you do not conceive you usually see a drop in your body temperatures on the day your period is going to arrive. This is due to the death of the corpus luteum and a dramatic drop in progesterone levels. It is important to remember that the days preceding ovulation are the days that you are fertile. By the time you confirm ovulation and get 3 high temperatures it is too late to conceive. Some woman can conceive the day following ovulation as the egg can stay viable for 24 hours, but the days leading up to ovulation are key when trying to conceive.

By charting your BBT along with your other fertile signs and when you have had sex, it can give you peace of mind that you have timed things correctly. You can then relax the rest of your cycle when you are in the so called ‘two week wait’ – the 14 days or so before your period is due. After you have confirmed ovulation you and your partner can stop having sex or can enjoy intercourse without the pressure of trying for a baby.

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