The basal body temperature (BBT) is the body’s lowest temperature during rest, and it is an important biomarker of a woman’s menstrual cycle. Tracking BBT is one of the essential components of sympto-thermal fertility awareness methods (FAM), which is a natural method of birth control that involves monitoring different fertility signs to determine the fertile and non-fertile days of a menstrual cycle.
How to track BBT
To track BBT accurately, it is important to use a special thermometer called a basal thermometer. This thermometer is more sensitive than a regular thermometer, and it can detect small changes in body temperature. Most thermometer detect temperature shifts up to one decimal point, a basal body temperature detects temperature up to 2 decimal places. ( I recommend this affordable Surgipack thermometer to all my Australian clients). This is important because internal temperature shifts are minor and can not be detected with a normal thermometer.
To take the temperature, a woman should attempt to wake up at the same time each day (setting an alarm may be necessary) and use the basal thermometer immediately upon waking up in the morning, before engaging in any activity, including getting out of bed, talking, eating, or drinking. The thermometer should be placed under the tongue as close as possible to the heat pockets in the tongue which are located on either side of the back molars. Other types of BBT thermometers can be placed in the vagina or rectum, but oral BBT thermometers are by far the easiest to use and the ones that most FAM methods advocate for.
What apps to use to use for fertility awareness
Women should record their temperature in a chart or a fertility tracking app without ovulation prediction algorithms, preferably at the same time each day. The best fertility tracking apps that I use and recommend are Lutea (this is a free charting app without prediction settings, it is user friendly but lacks many features ) and Read Your Body App (this app was designed by fertility awareness instructors and is the most comprehensive app for fertility charting. The cost is around $2 US dollars a month or equivalent in amount in different currencies). By tracking the BBT over several cycles, women can familiarise themselves with their individual pattern of temperature changes and record the most likely day of ovulation. It is important to remember that the timing of ovulation can change every single month and a woman should resist the urge to use algorithms and other calculations to try and predict fertile window patterns or the timing of ovulation. There are many other apps that can be used however most also include ovulation prediction algorithms which are not compatible with FAM. In all cases these setting should be switched off if possible. Here is a website with a full list of charting apps available https://fertilityawarenessmethodofbirthcontrol.com/fam-app-reviews/The practice of FAM helps identify fertility in real time and is not an ovulation prediction tool. Paper charting is also a viable option for women who do not like to use digital apps.
What you should know about BBT for confirming ovulation, predicting periods, and identifying hormone imbalances and pregnancy.
In a typical menstrual cycle, BBT is low during the first half of the cycle, which is known as the follicular phase. After ovulation, the hormone progesterone is released, which causes a rise in body temperature. This temperature rise indicates that ovulation has occurred, and the woman is now in the luteal phase, which is the period between ovulation and menstruation. To confirm ovulation, the woman should look for three consecutive days of temperature rise that are at least 0.2 to 0.5 degrees higher than the previous six days. This temperature rise is known as the “thermal shift,” and it confirms that ovulation has occurred. Average BBT before ovulation is 97°F or 36.1°C and 97.5°F or 36.4°C. After ovulation BBT increases to between 97.6°F or 36.5°C and 98.6°F 37°C. Basal body temperature will drop again if pregnancy doesn’t occur or remain high in case of pregnancy. Before ovulation there is sometimes a small dip in temperature at least 24 hours prior to ovulation. After ovulation temperature rises and remains high for at least 11 days (unless there is a luteal phase defect and a shorter luteal phase) before dropping again 1-2 days prior to the arrival of the next menstrual period.
It is important to know that the luteal phase can not last for longer than an absolute maximum of 17 days unless pregnancy has occurred. If pregnancy has occurred, basal body temperature will remain high. Most women with regular cycles have a luteal phase of around 14 days. Unlike the follicular phase which can change in length drastically each month, the luteal phase remains fixed. This means that if a woman usually has a luteal phase of 13 days, then each cycle, her luteal phase will likely be 13 days with only 1/2 a day – 1 day variance. If there is a change of days in the luteal phase, this can signify a hormonal imbalance (such as luteal phase defect) or transition into perimenopause. It is also important to know that the luteal phase should be at least 10 days in length to be considered “fertile.” If a luteal phase is less than 10 days, it does not allow an egg enough time to implant into the uterus and so pregnancy can not occur even if there is conception. Working on lengthening the luteal phase by boosting egg quality and progesterone levels becomes important for women who wish to get pregnant with a short luteal phase.
BBT can only confirm ovulation in retrospect, which means that the fertile window has already passed by the time the temperature rise is detected. Therefore, it is essential to track other fertility signs, such as cervical mucus, to identify the fertile window in advance. Once three consecutive days of higher temperature have passed, the woman is no longer fertile and can safely enjoy sex without the use of a barrier method. It is important to wait at least 3 days after the first temperature rise before engaging in unprotected sex (if wanting to avoid pregnancy). This is because it can take up to 3 days after the first temperature shift for ovulation to occur. In most cases ovulation occurs on the day of temperature rise or a day after, but because of the 3 day variance, most FAM methods advise a minimum of 3-4 days after the temperature shift before engaging in unprotected sex.
What can interfere with accurate temperature readings?
Several factors can interfere with accurate temperature measurements, including illness, alcohol, lack of sleep, medications, and other factors. Illness or fever can cause a temporary rise in body temperature, which can make it difficult to detect the thermal shift. Alcohol consumption can also affect BBT readings by causing dehydration and interfering with sleep quality. Lack of sleep or inconsistent sleep patterns can also affect temperature readings. A full 3-4 hours of uninterrupted sleep is required for accurate BBT readings. This makes it difficult to rely on temperature for women with insomnia, in post-partum with interrupted sleep from babies and night shift workers. Medications, such as thyroid hormones and some antidepressants, can also affect BBT readings. Other factors, such as stress, travel, or changes in routine, can also affect BBT measurements. If there is a known disturbance to a temperature reading such as alcohol or illness, the temperature for that day should be marked as “disturbed” in the menstrual chart or simply crossed out. Most apps will simply disregard this temperature reading for the day.
What alternative thermometers are there?
For women with chaotic schedules and interrupted sleep, devices like Tempdrop can be used instead of oral BBT thermometers. This device is worn on the upper arm and tracks proximal temperature when sleeping and gives an average BBT reading based on temperatures during sleep. It is ideal for shift workers and any one with interrupted sleep. Unfortunately the device is a financial investment which may be a barrier to women who are not sure if they want to commit to fertility awareness long term. With the rise in femm-tek, many new companies are woking on making temperature taking more convenient for women using new technologies that focus a lot on distal skin temperature readings. Some potential thermometers include the soon to be launched BBRing. Other companies such as Apple watch, Oura Ring and Fitbit which are predominately devices to measure sleep and fitness, are integrating temperature monitoring, however the reliability of these is currently questionable. It is important to remember that both proximal and distal skin temperature readings are not an accurate reflection of internal basal body temperature, however skin temperature readings still respond to changes in the menstrual cycle so can still be used for tracking fertility. Currently there is no adequate data to confirm the skin temperature recording in comparison to internal basal body temperature methods. However technology is evolving and new methods of measuring temperature for fertility awareness may prove to be a valid form of monitoring temperature. Read more about Thermometers and wearable temperature monitoring devices for fertility here)
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