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Basal Body Temperature
The Basal Body Temperature, or BBT for short, is the temperature of the body at rest for at least 3 hours of uninterrupted sleep. It is the lowest temperature of the body, or ‘waking temperature’, before it starts to rise with daily activities.
The significance of basal body temperature in relation to ovulation and the menstrual cycle, is that in an ovulatory cycle (a cycle in which ovulation occurs), the daily BBT chart shows a pattern of two phases (biphasic).
The first phase in an ovulatory cycle is the pre-ovulatory phase or follicular phase, before ovulation has occurred. The basal body temperature is lower in this phase of the cycle.
The second phase is the post-ovulatory phase or luteal phase, after ovulation has occurred. The basal body temperature is higher in this phase of the cycle.
Why Does the Basal Body Temperature Rise After Ovulation?
This rise in temperature is due to the heat-producing properties of the hormone progesterone. Progesterone is secreted by a temporary endocrine gland that develops after ovulation.
This temporary endocrine gland is called the corpus luteum. It forms from the remains of the ruptured follicle from which the egg was released upon ovulation.
Please note that basal body temperature will only be relevant to you where you are not taking hormonal birth control. There are different types of hormonal birth control but a number of them work by suppressing ovulation and shutting down the natural hormonal process of the ovulatory cycle.
The information I discuss below in regards to basal body temperature is based upon my own learnings from studies to become a Fertility Awareness Educator under the Symptothermal method. I am learning from the Natural Family Planning Teachers Association (NFPTA) UK.
Identifying A Thermal Shift Into the Post-Ovulatory Phase
To confirm ovulation and a transition into the post-ovulatory phase, we need to see a sustained temperature rise, also called the ‘thermal shift’.
The BBT chart will only move into this higher temperature phase after ovulation has occurred. The shift can only be identified retrospectively.
A sustained temperature rise must occur for three days before the thermal shift can be confirmed. There are further criteria that also need to be satisfied. Let’s break down the additional criteria:
1. Three days of undisturbed higher temperatures must be recorded above the previous six days. A horizontal line called the coverline is drawn on the chart to help visually separate the low-phase from the high-phase temperatures. This line is drawn above the highest of the six preceding low-phase temperatures;
2. On the third day of higher temperatures the temperature recorded must reach the full thermal shift level which is 0.2°C or 0.4°F above the coverline;
3. If the third day of higher temperatures does not reach the full thermal shift level then a 4th temperature reading must be taken into account. The 4th reading does not need to be at the full thermal shift but it must be above the highest of the preceding six days of lower temperatures.
If this is new to you, it may sound tricky and a little overwhelming. But with some practice and the guidance of an instructor, you will be able to do this easily within a few cycles.
Take a look at one of my charts below, where you can see the clear thermal shift, the coverline and the two distinct phases of lower temperatures and then higher temperatures.
Not Every Temperature Rise Indicates a Thermal Shift
Temperature readings can be impacted by a number of disturbances that may result in a false reading. Some common temperature disturbances include:
> Alcohol consumption the night before;
> Taking the temperature at a different time to usal;
> Fever due to illness;
> Travel especially through different time zones;
> Changing the thermometer you use;
> Changing where you take the temperature from (e.g. switching from oral to vaginal or simply just a different placement of the thermometer)
Any disturbed temperatures should be noted on the chart. Disturbed temperatures must not be included in the six lower-phase temperatures or the three higher temperatures.
Basal Body Temperature and the Fertile Window
It’s important to understand that basal body temperature can only be used to determine the end of the fertile window. Why? Because the shift to the higher temperature phase only happens once ovulation has already occurred.
To determine the start of the fertile window, we can observe cervical mucus. This is beyond the scope of this post, but you can click this link to read more about cervical mucus and other signs of fertility.
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What Can You Learn From Your BBT Chart
The information you can obtain from charting your basal body temperature includes:
1. Confirmation that ovulation has occurred. BBT does not predict the day of ovulation, but provides evidence of ovulation after it has occurred.
2. Confirmation that ovulation has not occurred. A temperature chart where there is no shift into a higher range of temperatures may indicate that ovulation has not occurred. This could indicate an anovulatory cycle which may happen from time to time or an underlying condition for investigation by a medical professional.
The normal functioning of your ovulatory cycle is an indicator of overall health, so if ovulation is not happening then this is something worth investigating. However, first you should ensure you are taking your temperature correctly. Working with a Fertility Awareness instructor can help with this or you can self-teach with a book like Taking Charge of Your Fertility, The Fifth Vital Sign or the Sensiplan Manual.
3. Confirmation of pregnancy. A BBT chart which shows a temperature that remains high for 20 days can confirm pregnancy. The corpus luteum producing progesterone supports the pregnancy for 8 weeks after conception until the placenta can take over and produce the hormones required.
Basal Body Temperature and Your Period
One of my favourite uses of tracking my basal body temperature is the information it gives me about when to expect my next period. Forget relying on an app that uses an algorithm to predict when your next period is due, your body has its own inbuilt algorithm!
“Forget relying on an app that uses an algorithm to predict when your next period is due, your body has its own inbuilt algorithm!”
Unless pregnancy occurs, the post-ovulatory phase of your cycle has a finite life-span, driven by the finite life-span of the corpus luteum. When no pregnancy occurs after ovulation, the corpus luteum will start to regress after about 7 days and it will completely disintegrate within 10 – 16 days (usually around 14 days).
While the phase from menstruation until ovulation could be any number of days, weeks or months depending how irregular a cycle is, once ovulation occurs, the post-ovulatory phase is always going to end within 16 days, unless pregnancy occurs.
You can use this information to know when your next period will arrive. Once you have confirmed ovulation by a sustained temperature shift, you know your period will arrive in about two weeks time. This is further confirmed by a temperature drop that will usually occur just prior to the arrival of your period.
These indicators are more accurate than any app is going to be.
History of BBT Charts
In case you’d like a bit of background information about basal body temperature, I’ve included some historical information below.
Evidence of the biphasic temperature pattern occurring in women was first noted by Dr W. Squires in a London Tuberculosis Sanatorium in 1868 and also by Dr Mary Putnam in New York in 1875. The both described the rise in temperatures that occurred approximately half-way through the menstrual cycle, but neither connected the finding to ovulation.
Van de Velde, a gynaecologist in Holland, later made two important observations regarding the basal body temperature patterns of the menstrual cycle. In 1904 he identified the rise from a lower BBT to a higher BBT was related to ovulation. And in 1928 he identified that this rise was due to the hormone progesterone secreted by the Corpus Luteum.
In 1968 Dr John Marshall, a neurologist in the UK, performed the first prospective trial of the Basal Body Temperature Method and published these studies and a book.
The Better Period
Ellie Heasman is a period blogger and founder of The Better Period. Ellie helps people on their journey to a better period through introducing them to the world of menstrual cups and period underwear, and sharing knowledge about the menstrual cycle and fertility awareness. You can join in the better period conversations on Instagram @thebetterperiod_ or find out more about Ellie here.