Before trying for a baby, the chances are you’d never even heard of BBT (basal body temperature), but if you’ve been trying for a few months and are still waiting for that little blue line, a few google searches will tell you that tracking your temperature in this specialised way could give you vital clues about when your body is most likely to conceive.

What is BBT?

A basal body thermometer will track tiny fluctuations that occur in your body temperature when it’s at its lowest, following a night’s sleep.  At the beginning of your menstrual cycle, your temperature will be lower – this is known as the follicular stage.  Then as your oestrogen levels rise, triggering ovulation, and the production of progesterone, your temperature will increase by a tiny amount (from 0.5°F-1.0°F or 0.25° to 0.5°C).  By charting your BBT each day, you can look out for this temperature increase and get a good idea of exactly when you’re ovulating.

Basal Body TemperatureHow do I chart my BBT

Many BBT thermometers will come with a chart (it’s a good idea to make some copies as you’re likely to be doing this for a few months).  There are of course apps too that you can download to keep a record of your temperature readings.

It’s important to take your temperature at around the same time each day, before you get out of bed and although you can take it orally, vaginally or rectally make sure you stick to the same method each time.

What am I looking for?

A sudden increase in temperature (around 0.2°C) will suggest that you’re ovulating.  Your most fertile day will be the day of the temperature spike and the few days beforehand, so the BBT is a really good way of establishing a pattern for future months, rather than the present month.

At the same time, you may notice a change in your cervical mucus.  As yuck as this sounds, it’s a really good indicator that your body is about to ovulate.  The consistency of the mucus changes throughout the menstrual cycle and upon ovulation it will become more jelly-like, resembling raw egg whites in colour and consistency.  This change makes it easier for sperm to reach the egg.

BBT and pregnancy prediction

BBT is a common method for predicting ovulation, but it can also be used to detect the beginning of a pregnancy too.  The two things to look out for on your BBT chart are an implantation dip, about a week after ovulation which lasts about a day.  There’s also a triphasic temperature pattern which may follow; an increase in temperature about a week after ovulation.  Both of these signs may hint at a pregnancy, but are by no means a guarantee.

Another element of your BBT chart to look out for is the length of your ‘luteal phase’.  This is the period between ovulation and menstruation.  If you see that your luteal phase has gone at least one day past the usual length, you might be pregnant. If it goes two days past the longest luteal phase you’ve ever had, the likelihood of being pregnant is even higher. This is definitely a good time to get a pregnancy test!