If you are a person who takes their basal body temperature as a form of contraception or to predict ovulation, you probably wonder why some of the temperature variations occur. In some cases, there is more than one cause, so interpreting the results can be confusing.
For example, a drop in your basal body temperature after ovulation can have more than one explanation. Some people say that a one-day dip occurs right around the time a fertilized egg starts making a home in the lining of their uterus (implantation). Others say this dip is simply a random fluctuation that means nothing. So what are you supposed to do with this information?
To help you understand what your body is up to, it is important to understand exactly what BBT is and then make sure that you track your BBT accurately. Then you can start to interpret the results in a more meaningful way.
What Is Basal Body Temperature?
You may have grown up hearing that a normal body temperature is 98.6 degrees Fahrenheit. While that number works as a general standard, most people don't measure 98.6F throughout the day. Your temperature makes small adjustments constantly. Your basal body temperature, or BBT, is the range of numbers you see on your thermometer throughout the day when you are at rest.
Your BBT fluctuates in response to several factors, but hormones like estrogen and progesterone have a big impact. According to the National Institutes of Health (NIH), higher estrogen levels prompt your BBT to go down, while progesterone prompts it to go up.
Charting your basal body temperature offers a cheap and easy way to find out if you tend to have a drop in basal body temperature after ovulation. This knowledge can help you when you are trying to get pregnant. It can also help you to avoid pregnancy. But you need to track your BBT for some time before the information becomes useful.
How to Track Basal Body Temperature
To accurately interpret changes in your BBT, you will need to keep records for at least three months. After three months, you may begin to see a pattern that will show when you ovulate and when you don't. You will also be able to see any irregularities (like a one-day dip) in your cycle.
Gather Tools to Track BBT
While you can track your basal body temperature on a regular sheet of paper, you may find it simpler to use a downloadable BBT chart like this one:
In addition to your chart, you will need to purchase a thermometer that measures your temperature to a tenth degree. Think 97.1 degrees instead of 97. This helps you stay precise while charting your BBT and more easily see changes. Most digital thermometers display this level of detail.
Keep your thermometer next to your bed, along with a pen and your BBT chart for the month. Then follow these instructions in order:
- Each morning when you wake, before doing anything else, take your temperature. You can do this orally, under your arm, or rectally, but whichever way you choose, stick with the same method every time.
- Mark your chart with a dot to indicate your temperature that day.
- After several days, you can begin to connect the dots and see how your temperature fluctuates from day to day. These fluctuations will be small until you are ready to ovulate.
A rise of 0.5 to 1 degree F or higher for three or more days in a row usually indicates that you have ovulated, according to the NIH. This may sound like a tiny temperature change, but on your BBT chart, it will look big. Some women might also notice a dip in temperature right after ovulation and might wonder what it means.
What Does a Drop in BBT After Ovulation Mean?
Typically, after you ovulate, your BBT stays elevated for several days. Once you start your period, your BBT will probably drop and stay down for however long your period lasts. So what does it mean if your BBT dips for just one day and well before your period is due to start?
If a dip like this occurs, it usually happens between ovulation and when you start your period (called the luteal phase of your cycle). Information about how to interpret this dip is widely available but can be terribly confusing. There are many claims and caveats flying around the internet and not all of them are backed by strong scientific evidence.
Pregnant vs. Not Pregnant
One source of information about basal body temperature is the fertility industry. Companies selling fertility apps or products sometimes provide consumers with information about how to interpret temperature changes.
For example, a free fertility app compiled data on women's BBT readings and pregnancy results. They found that pregnant people were twice as likely to show a BBT dip after ovulation as those who were not pregnant. However, while these results are compelling, they were not discovered in a peer-reviewed scientific study. Published peer-reviewed studies put various safeguards in place to ensure accurate results. Without these safeguards, it is hard to know for sure if the findings are reliable or definitive.
And the results of that study contradict findings by the NIH. Health experts at NIH explain that your BBT should actually increase and stay high if you are pregnant. "A basal body temperature elevation that does not return to baseline with expected ovulation could be an early indication of pregnancy," they report.
Other Potential Causes
Of course, other factors can affect your temperature. For instance, any of these circumstances can cause your BBT to change:
- Alcohol consumption
- Environmental factors like the temperature of your home
- Illness and infection can raise your temperature through fever
- How well you sleep
- Medications that include hormones, like birth control or hormone replacement therapy
The different opinions about a dip in BBT after ovulation and the number of factors involved can easily throw you into confusion if you are trying to use the information to become pregnant. But the bottom line is this: Some people experience a slight dip in BBT during early pregnancy, but many don't. So, while BBT can indicate early pregnancy, you can't use it to know for sure.
Is Charting BBT Worth It?
Conceiving a baby can be a long frustrating process. If you want to try to keep a chart of your basal body temperature, it can provide a way to take action in the midst of all the waiting. It can also give you and your healthcare provider a valuable glimpse into your body's fertility process.
If you feel concerned about your ability to conceive, or confused about what to do, give your provider a call or make an appointment. Just remember, you are not alone!
If you'd like to learn more information about fertility or join a fertility support group, take a look at these options: