Understanding Basal Body Temperature Charts for Pregnancy Tracking

A basal body temperature chart is a great way to learn the following:

• Understand your menstrual cycle better
• Determine the day of ovulation
• Find out what your most fertile days of the month are.

Any basal body temperature chart that shows ovulation is biphasic. To differentiate, the word bi   means   two and  phasic  means relating to a phase.     

There are two distinct temperature phases in the BBT ovulation chart – one before ovulation and one after ovulation.

If you look at the sample chart in the image above, it’s clear that temperatures before day 15 are generally lower than temperatures after day 15. For this example chart, we know that ovulation occurred on day 15.

A triphasic chart is a basal body temperature (BBT) chart with three distinct temperature rises. This pattern is considered a possible sign of early pregnancy. Seeing this pattern on your chart can lead to hope for a positive pregnancy test and even a sudden awareness of other possible early pregnancy symptoms.

   There are  three temperature changes in a triphasic chart  .  For the chart to be truly triphasic, this third temperature change must occur at least seven days after ovulation. 

Look at the sample chart above. Do you see the third temperature change starting on day 25? This change occurred ten days after ovulation. However, even if it started a little earlier—say, just seven days after ovulation—it can still indicate that the chart is showing a triphasic pattern.

A triphasic rise in temperature can indicate pregnancy, but it doesn’t always. And you can get pregnant without seeing a triphasic pattern.

The first step to charting your basal body temperature is to create a chart to record your body temperature. You can   find   sample charts in some  fertility books, such as this one, which many people consider the primary source of guidance for charting basal body temperature.  

Another option for charting is fertility awareness software, also known as a fertility calendar. There are several fertility calendar options online and several apps for your phone. Many of them are free.

You can also make your own chart. If you make your own, you want to plot the temperature along the vertical and let it be one tenth of a degree for each square. Along the horizontal line, you will have the days of your cycle.

Most people prefer to use digital tools because you can record a lot of information and reduce the risk of human error. Most ovulation programs automatically show when ovulation occurs. If you try to chart your temperature yourself, you may be worried that you will make a mistake.

Once you have something to record your body temperature, it’s time to take your basal body temperature. You’ll need a thermometer—any store-bought option will work. There are a few important guidelines you should follow when using your BBT.

Take your temperature at the same time every morning (plus or minus 30 minutes). For example, if you take it at 7:30 a.m. every weekday, take it no earlier than 7:00 a.m. or later than 8:00 a.m. on other days.

Avoid getting up, sitting, walking, or going to the bathroom before taking your temperature. You should put the thermometer in your mouth a minute after waking up.

Before you take your temperature in the morning, make sure you’ve had at least three to four hours of uninterrupted sleep. If you’ve been up all night or wake up frequently during the night to walk, this will throw off your results.

Fertility Friend, a free online fertility charting software company, conducted an informal analysis of basal body temperature charts on its site to see if a triphasic pattern could indicate pregnancy. This old study was by no means a peer-reviewed scientific study, but the results are interesting.

In their informal analysis, they considered a triphasic pattern to be a second significant upward temperature shift of at least 0.3 degrees Fahrenheit that occurred at least 7 days after ovulation.

In practice, there is no definitive definition of a three-phase diagram. People comparing and sharing graphs may disagree on whether a particular pattern qualifies as a three-phase diagram. The definition provided here is for analysis purposes only.

 Based on this data  , it is therefore three times more likely that a chart showing a triphasic pattern    will belong to someone who is pregnant. 

In case you missed it, I should point out a very important fact here: While 12% of pregnancy charts had a triphasic pattern, 88% did not.

Or to put it another way, if you look at the BBT charts of 100 pregnant people, only 12 of them will show a triphasic pattern. However, if you don’t see this pattern, it doesn’t mean you’re not pregnant. It’s also important to remember that a triphasic chart   doesn’t always mean you’re  pregnant  .  

Basal body temperature can reveal interesting information about pregnancy. When you are not pregnant, progesterone levels drop, which causes your basal body temperature to drop. However, pregnant people usually have increased progesterone levels to help with pregnancy. Therefore, a persistent increase in BBT can indicate pregnancy. However, everyone is different, and some pregnant women may not experience an increase in their basal body temperature. In addition to the triphasic pattern, here are three other ways a BBT chart can indicate pregnancy or the possibility of pregnancy.

A BBT chart can help you determine whether you had sex on your most fertile days. Although your basal body temperature cannot predict ovulation,    your BBT chart   can help you determine    whether you ovulated a few days after it occurred.

This means you don’t have to know if you had sex on the “right days” until after you ovulate. But you can look at your chart and determine that. If you have sex two days before you ovulate, you’re most likely to get pregnant.

An implantation dip is a one-day drop in temperature about a week after ovulation. An implantation dip is usually nothing more than a mid-cycle drop in temperature and does not indicate pregnancy. Whether this is a possible sign of early pregnancy is debatable.

The most reliable way to detect pregnancy on a BBT chart requires patience. The old way to do it involves waiting to see if your luteal phase — the time between ovulation and your expected period — is longer than usual.

For most people, the luteal phase does not vary by more than a day or two from month to month, even though the length of their menstrual cycle varies. For example, the length of a cycle may be between 30 and 35 days, but the luteal phase is consistently 12 or 13 days.

If you notice that your luteal phase is at least one day longer than usual, you may be pregnant. If it’s been two days since your longest luteal phase, your chances of getting pregnant are even higher. It’s a good time to take a pregnancy test.

If it’s been 18 days since you ovulated and you still haven’t gotten your period, there’s a very good chance you’re pregnant. Few people can go that long without taking a pregnancy test. However, this is the strongest early sign of pregnancy that can be detected using a BBT chart.