Basal body temperature (BBT) tells you a lot about what’s happening in your reproductive system, from when your period is due to when you’re ovulating. That makes BBT measurement very useful when you’re trying to get pregnant or avoid getting pregnant, for that matter.
However, if you don’t measure it correctly, or you can’t chart it properly, that throws the whole thing off! Let’s look at the proper way to measure your BBT and keep track on graphs, and what this tells you about your fertility.
What is basal body temperature (BBT)?
Basal body temperature (BBT) refers to the human body temperature at rest, expending the minimum amount of energy. The easiest time to measure this is right after you wake up in the morning. Since it’s only looking at the bare minimum of metabolic functions, it’s quite different to measuring your usual body temperature.
How to measure basal body temperature (BBT)
Your body temperature rises above the BBT with even the slightest movement, so the best time to measure it is before you even get out of bed in the morning. Keep the points below in mind when you measure your BBT. And remember – when you make a record of your BBT, take note of any physical changes in terms of how you’re feeling as well so that you can track how your body changes over your cycle.
1. Use a BBT thermometer
Measuring your BBT is different to just checking if you have a fever, so you’re going to need a more accurate thermometer. After ovulation, your BBT will only rise by about 0.3 to 0.6 of a degree, so you’ll need to measure your temperature to at least two decimal spaces to get an accurate reading. It could be useful to invest in a digital thermometer or BBT thermometer. The more accurately you can measure your BBT, the better you’ll be able to track your cycle.
2. Check your BBT every day when you wake up
BBT refers to your body temperature at rest, so in theory, the ideal time to measure it would be while you’re asleep. For obvious reasons, it’s a bit tricky to measure your own temperature while you’re still asleep.
But, you can do the next best thing by measuring your BBT at the same time every morning when you wake up, but before you get out of bed. It may be helpful to set an alarm clock to help you get into the habit of measuring it at the same time every day. Keep your BBT chart and thermometer on your bedside table for easy access!
3. Measure your BBT from the very back of your tongue
If you’re using an oral thermometer, make sure it sits on the underside of your tongue all the way at the back. There can be differences of as much as a tenth of a degree depending on where you place the thermometer in your mouth, so make sure you use the same spot every day and don’t let it move around while you’re measuring. If you keep your mouth closed so air can’t enter and don’t move, you’ll be able to get the most accurate reading possible.
4. Don’t do anything before measuring your BBT
Even a yawn can be enough to raise your temperature above BBT levels, so make sure you stay in bed and keep movement to an absolute minimum before you measure it in the morning. Make sure you measure your BBT before anything else: that means no getting up, no eating, no drinking, and try to avoid movement as much as possible.
5. Be careful of sleep deprivation and alcohol
Factors such as sleep deprivation and alcohol alter your body’s thermoregulation systems, which means your body temperature ends up lower than it would otherwise be. This means that if you drink alcohol or don’t get enough sleep the night before, it could be harder to get an accurate BBT reading in the morning.
What do changes in basal body temperature indicate?
If you chart your BBT each day, you’ll be able to make a line graph of the results. Ideally, you’ll be able to see a biphasic pattern: that is, two distinct periods of lower temperatures and higher temperatures.
Your baseline BBT depends on individual factors, but for most women, it sits around 97.0℉ to 97.5℉ before ovulation, and bounces up to between 97.6℉ to 98.6℉ afterwards. This two-phase pattern corresponds to the menstrual period, follicular phase, ovulation, and the luteal phase of your cycle. So, what does this mean for you if you’re trying to get pregnant?
How to read a basal body temperature chart?
What a BBT chart can tell you
- Your most fertile days
- When your next period is due
- What your general health is
- Whether you are pregnant
What to look for on your BBT chart
- What day your BBT drops
- How long your luteal phase lasts (the high-temperature phase)
- How long your follicular phase lasts (the low-temperature phase)
- The difference between your highest and lowest BBT levels
- Your average temperature in the follicular phase
Measuring your BBT like this will also help you work out when your period is due, if your menstrual cycle is regular. If your BBT is high for 14 days and then drops, it usually suggests that your period is due that day.
What does a basal body temperature chart tell me?
The biphasic pattern in your BBT over a cycle of 25 to 38 days tells you when you are most likely to fall pregnant. Your BBT is higher during the luteal phase and lower during the follicular phase followed by a sharp rise when you ovulate. Your most fertile days are the 5 days before and the 24 hours after ovulation.
If your luteal phase or follicular phase is too long or too short, or if there are irregularities in the temperature pattern, it could point to something a little out of whack in your fertility cycle. Let’s look at some of the specific things patterns in your temperature cycle can indicate.
What if my luteal phase is too long?
The luteal phase of high temperature normally lasts 12 to 17 days. If your BBT rises and then stays high for 18 days or more, it’s likely that you’re pregnant. If your high temperatures continue past the day your period is due, wait one week and take a pregnancy test. If it’s positive, get yourself to an OB-GYN!
However, if you keep getting a negative on pregnancy test kits, an overly long period of high temperatures could point to an elevated progesterone level. The causes for this aren’t quite clear, but it’s believed to be linked to sex hormone imbalances. See your doctor to work out what you can do about it.
What if my luteal phase is too short?
Luteal phases are judged as short if they last less 10 days or less. This usually means that the body isn’t producing enough of the hormone progesterone, which can result in a condition called a luteal phase defect. When this happens, it’s harder to get pregnant, and harder for a fertilised egg cells to implant into the endometrium. This is a potential cause of infertility.
What if my follicular phase is too long?
The average luteal phase lasts about 14 days, and this is generally consistent between your cycles. On the other hand, the follicular phase is a little more changeable. If your cycle is 28 days, then it should last around 14 days, but if your cycle is closer to 38 days, then it can last as long as 24.
If your follicular phase is too long, or your menstrual cycle lasts 39 days or more, this suggests oligomenorrhea, meaning your ovulation cycle is off-kilter. This could point to conditions such as PCOS (Polycystic Ovarian Syndrome) or early menopause. This can be an issue in infertility, so discuss this with a doctor or OB-GYN to work out what you can do about it.
What if my follicular phase is too short?
If your follicular phase lasts 10 days or fewer, your menstrual cycle could be irregular. In this case, it’s thought that the short period of low temperatures could be caused by conditions like irregular ovulation or a lack of ovulation, luteal phase defects, or perimenopause.
Follicular phase shortening could point to something unusual in the way your egg follicles are maturing for ovulation. Left alone this could be a factor in infertility, so if you’re concerned, it’s best to see a doctor and investigate it.
What if my BBT stays low over the whole cycle?
If your BBT doesn’t rise over the course of the cycle, but just stays low the whole time, it could indicate that you’re not ovulating even if your menstrual period is arriving as normal. This issue is easy to miss unless you measure your BBT. In this case, it’s best to see an OB-GYN to work out the cause.
What if my BBT fluctuates a lot?
The difference between your BBT in the follicular and luteal phases varies between individuals, but it should still form a neat two-phase pattern on a line graph – not bouncing up and down all over the place. If this happens, it could be a result of stress pushing your hormone balance off-kilter. On the other hand, it could also point to some ovarian dysfunction, PCOS, or high prolactin levels. Either way, it’s worth getting it checked out by a doctor.
What if my BBT drops suddenly in the middle of the luteal phase?
If your BBT drops suddenly for a day or two during a period of higher temperatures and then rebounds, you could be witnessing the tell-tale “implantation dip” – a very early sign of pregnancy. If the high temperature period continues for another two weeks or more, you could be pregnant, so take a test to confirm.
What if my BBT drops suddenly during pregnancy?
A sudden drop in BBT while you’re pregnant could be a sign of miscarriage. If your BBT drops and continues to remain lower range after you’ve confirmed a pregnancy, see a doctor to work out what’s happening.
What if my BBT is less than 96.8 during the follicular phase?
If your BBT drops below 96.8℉ during the lower temperature follicular phase, it could be a sign your body isn’t keeping itself warm enough. This could be a sign of some lifestyle habits that are making it harder for you to fall pregnant, like being underweight, eating disorders, drinking too much alcohol, or not getting enough sleep.
Make sure you’re leading a healthy, active lifestyle and try to stay on top of stress. This should help keep your body’s thermoregulation system in check.
Stay fertility-aware by tracking your BBT!
Measuring your basal body temperature doesn’t just tell you when you’re due to ovulate. It can indicate whether your ovulation is happening on schedule, or point at some health conditions that affect women’s reproductive organs. If you can get into the habit of measuring and recording your BBT accurately each morning, you’ll notice anything out of the ordinary right away.
The more you know about your health, the better care you’ll be able to take of your body in your daily life – which means when your most fertile days arrive, you’ll be well prepared!