BBT (Basal Body Temperature) - What do we chart and why?

What is BBT charting?

Basal Body Temperature Charting has long been used to help track menstrual health, fertility and ovulation. Basal body temperature charting is measuring your temperature in the early morning, before you get up (even before you go for a quick morning wee!). The idea is to catch your temperature at its lowest, or base. 

This is important when tracking fertility as your base temperature will hopefully have a pattern that it follows throughout the month, a pattern that will help you spot your fertile window, ovulation and possibly the onset of your menstrual cycle too. These patterns can show when you’re fertile, when you’ve ovulated and then when you’re in your ‘safe zone’.

When charting BBT it’s important to use a thermometer that allows you to measure to 1/10th (98.6) of a degree in Fahrenheit or 1/100th (37.00) of a degree in Celsius. Your temperature doesn’t fluctuate too much in an overall cycle, so you need to be able to see your temperature changes in detail. 

Who uses BBT charting for contraception?

You can use BBT charting as part of your natural birth control to avoid pregnancy. If you know when your fertile window will be, it’s easy to use protection or abstain from sex during this time. By measuring your BBT alongside tracking symptoms like cervical mucus and getting to know your cycle you can more accurately spot your fertile window. 

Who uses BBT charting to help with conception?

To increase the chances of pregnancy you need to pinpoint your fertile window and make sure you have sex during this time. This helps to ensure that there are sperm waiting for the egg when it’s released. We can spot ovulation in changes in your BBT just before and then after ovulation. 

What should BBT look like in the follicular phase?

The follicular phase of your cycle is the first part of your cycle. This phase varies from person to person as it starts on day one of your period and finishes on the day you ovulate. In the follicular phase your temperature should remain relatively steady. Most women find they measure between 97.51 and 98.24 fahrenheit (36.39 and 36.81 celsius) during this stage.

Ovulation can happen as early as cycle day 7 for women who have very short follicular phases and therefore normally short cycles too. 

What should BBT look like around ovulation?

Just before ovulation, you may notice a small dip in your BBT, but this isn’t always the case. The rise in BBT after ovulation is what we’re really looking for. This is confirmed when you see 3 consecutively higher temperatures after the previous lower 6… sounds complicated but don’t worry, it’s not. Essentially, you should see a rise in your BBT that lasts at least 10 days, this should be higher than the 6 last measurements you took before the rise. The increase in temperature is very small, usually around 0.4F (0.2C), but it’s easy to see when you use an app like ours. This rise in temperature is due to an increase in progesterone which happens after the egg is released. 

Does BBT rise straight after ovulation?

Most people think that BBT rises straight after ovulation. This isn’t strictly true. Some women experience a shift in BBT that takes 2-3 days to reach the temperatures it will remain at for the luteal phase. This is why it’s always a good idea to combine BBT with hormone and symptom tracking to get a much clearer idea of when your fertile window is.

What should BBT look like in the luteal phase 

The luteal phase is the stage of your cycle that comes after ovulation and up to the first day of your next period. Ovulation usually takes place 10 - 16 days before your period. This post-ovulation phase is called the Luteal phase, which usually remains stable in length of each cycle. 

In the luteal phase, your BBT should remain pretty consistently higher than your follicular phase. The rise in temperature is typically around 0.4 degrees Fahrenheit or 0.2 degrees Celsius, that said, the rise can also be as small as 0.2 degrees Fahrenheit or 0.1 degrees Celsius. Your BBT should remain at this raised level until you get nearer the start of your period. At the end of this phase, you will see your BBT starts to drop, this usually indicates a drop in progesterone and the onset of your period. 

Luteal phase defect

If your luteal phase is less than 11 days long you might have a luteal phase defect. What do we mean by this? If there are 11 days or less after ovulation and before your period starts, it could indicate that your body isn’t creating enough progesterone or that the lining of your uterus isn’t responding to the increase in progesterone. This could harm your chances of conceiving.

What else could be affecting your BBT? 

One key thing to remember when using BBT charting is that you have to be extremely consistent with your measurements. You need to measure at more or less the same time every day, before you’ve sat up or moved around - no going for a wee and then taking your temperature! You need to have been asleep for at least 4 hours too. This is because you need to capture your temperature at it’s complete lowest, and these things can affect that.

Other things that can affect your BBT are illness, emotional and physical stress, medication, alcohol and travel. This is why we suggest you track BBT alongside other symptoms like cervical mucus and use LH monitoring too.

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