Basal body temperature - how to chart ovulation
When you're trying to conceive, it's best to have sex every two to three days to increase the chance that there will be sperm in the right place at the right time.
The National Institute for Health and Clinical Excellence (NICE) recommends avoiding charting ovulation as it can add unnecessary stress.
But if your periods are irregular, sex every few days isn't possible, or you're using assisted conception, then charting your basal body temperature (BBT) will give you a good idea of exactly when you are at your most fertile by showing you when you are ovulating.
What is your basal body temperature?
The basal body temperature is the lowest temperature of your body at rest. It will change throughout the month and is affected by ovulation, which makes it increase by 0.5°F to 1°F (0.25°C to 0.5°C).
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There is also a tendency for the body temperature to be lower in the days before ovulation and higher afterwards. This divides your menstrual cycle in two, with a peak in the middle when you ovulate.
The phase before ovulation is the follicular stage and the stage after is the luteal stage.
The higher temperature is caused by oestrogen. After ovulation, there is a rise in progesterone, which is released from the corpus luteum that develops from an ovarian follicle.
If a pregnancy occurs, the corpus luteum takes on a temporary role of maintaining pregnancy hormones until the placenta takes over. But if there's no pregnancy, the corpus luteum breaks down and disintegrates at the same time as menstruation, when your BBT rises again.
Your cycle may be the same each month or might vary wildly. By charting your temperature you can get to know what is happening during your cycles and also see whether your luteal phase is long enough to sustain pregnancy.
How to chart your basal body temperature
You will need a digital basal body temperature thermometer (which is more sensitive than your run-of–the-mill kitchen cabinet thermometer as it has a finer scale and will detect subtle changes). You can buy one of these for under a fiver and they often come with blank charts to fill in your results. If not, there are charts online you can print off.
- Establish a routine
You need to take your temperature when your body has been at rest for at least three hours, so the best time to take it is when you wake up and before you have eaten or exercised. Keep your charting kit next to the bed and take the reading from there. It's best to keep to the same time every day.
- Stick to the same method of recording temperature
You can use the thermometer orally, vaginally or rectally, but stick to the same method throughout the month. It only takes a couple of minutes to get an accurate reading. Try to make everything as uniform as possible. Do this every day and plot the temperature on your chart.
- What to look out for
If you notice a rise of 0.2°C when compared with the previous six days, this is the time to have sex. The spike in temperature tells you that you have already ovulated. If you can detect a pattern over a few months of charting, you'll be able to predict when you will ovulate the next month and you can then try to have sex two or three days before (which is when you have the most chance of conceiving) as well as during and after so as to target the released egg with as much sperm as possible.
- Keep an eye on your cervical mucus
Vaginal secretions are one of the key signs of ovulation. The cervical mucus produced just before ovulation is sometimes known as egg white cervical mucus (EWCM) because you may be able to stretch an unbroken strand between your fingers and its consistency is similar to raw egg whites. (If you've got raw egg phobia, this won't be the method for you.) But if you do this alongside your chart, you will begin to recognise the signs of ovulation. Typically, there will be a lot more mucus when you ovulate to help the sperm get to their destination.
- Analyse your charts for any patterns
The biggest variations should be in the follicular stage, while the luteal stage should be fairly constant (an average of 14 days). A short luteal phase (less than 12 days from ovulation to menstruation) may be causing infertility or early loss of pregnancy.
What Mumsnetters say about taking your basal body temperature
- I measure mine. It's not so bad, once the alarm goes off I grope in the dark for the thermometer, shove it in my mouth, switch the alarm off, go back to sleep and read it later in the day. Peachygirl
- You have to be OK with observing your physical signs quite closely, so not at all squeamish about checking your cervical fluid / position of cervix etc; and you also need to have a relatively set routine so that you can take your waking temperature at about the same time every morning. kalidasa
Last updated: 11 months ago