Basal Body Temperature (BBT) gives you a rough indication of when ovulation has occurred – i.e. it tells you that your fertile window has passed, that you have ovulated and are now in the second, luteal, phase of your cycle (or the two week wait, often abbreviated to 2WW). Although measuring your BBT is not recommend by the National Institute for Clinical Excellence (NICE) who say that “The use of basal body temperature charts to confirm ovulation does not reliably predict ovulation and is not recommended” many do find it a good introduction to beginning to understand their cycle better.
Rising progesterone levels at this time cause a subtle, but detectable rise in your BBT. Tracking your BBT over several cycles can give you a clearer indication of when you are likely to ovulate in future cycles and whether or not you are ovulating each cycle or not, although this is not always the case for all. Charting your temperature can also indicate whether you are having healthy ‘normal’ length cycles, for instance, if your luteal phase is shorter than 10 days this is not likely to be long enough to sustain a pregnancy.
In order to accurately check your BBT you will need a special BBT thermometer, which measures your temperature with a greater degree of accuracy than a ‘normal’ temperature. These are available cheaply from most high street chemists.
Begin by checking your temperature from day one of your cycle. You will need to check your temperature as soon as you wake in the morning, before you do anything else, before having a drink or getting out of bed etc. This is so that you are getting an accurate and true reading of your basal temperature and not measuring fluctuations caused by movement and hot drink and food. Ideally, you need to check your temperature at the same time each morning. Your BBT can also fluctuate with the time of day. It is also important that you use the same method – some women find that they get more accurate, stable results by checking their temperature vaginally, instead of orally for example.
Once you have checked your temperature, you will need to mark it on a chart, these often come free with a BBT thermometer, or you can find one to print easily on the internet. There may be fluctuations in your readings, but a consistent increase of 0.2 degrees centigrade compared to the preceding six days, which is then sustained for at least three days indicates that ovulation has occurred. Once this happens the chance for conceiving in this cycle has now passed.
The chart below indicates when ovulation has most likely occurred. Note the thermal shift is sustained for the whole of the 2WW, until the end, when progesterone levels fall and menses (fresh bleeding) occurs again.
If pregnancy occurs, then the thermal shift is likely to be sustained as the progesterone levels remain elevated.
Other factors that can cause fluctuations in BBT include alcohol consumption, disturbed sleep, shift working, or illness. If you experience these it doesn’t mean that you cannot chart, or that if you have one or two odd temperatures that your chart cannot be interpreted properly. It is important, however, to bear in mind that these factors might influence the pattern of your chart and that charting alone is not an accurate indicator of fertility.
Occasionally, a dip in temperature can be seen at around 8/9 days post ovulation (DPO). This can correlate to implantation of a fertilised egg. However, this does not always happen with every pregnancy and absence of an implantation dip does not indicate that you have not become pregnant that cycle. Equally, an ‘implantation dip; can appear to be part of a chart, and yet pregnancy has not occurred. Other temperature patterns during a 2WW include a ‘triphasic’ temperature pattern, where a second thermal shift can be detected to occur around 7-9 DPO. This again has an association with pregnancy, but is not a definite indication that you are or are not pregnant.
by Lorraine Berry – Midwife