How to Detect Ovulation and Early Pregnancy With a Basal Body Temperature Chart

You can use a basal body temperature (BBT) chart to conceive faster by determining your most fertile days. Detecting ovulation with BBT charting is relatively easy, noninvasive, and inexpensive. Your gynecologist or reproductive endocrinologist may recommend charting to help detect when ovulation is happening or to get a better idea of your menstrual cycle patterns.

There are many advantages of BBT charting. Here’s everything you could want to know about basal body temperature charting.

What Are Basal Body Temperature Charts?

Basal body temperature charting is an excellent way to learn the following:

• Get to know your menstrual cycle better
• Detect ovulation day
• Learn what your most fertile days of the month are

Biphasic chart

Every basal body temperature chart that indicates ovulation is biphasic. To break down the word, bi means two, and phasic means related to a phase.

On a BBT chart with ovulation, there are two distinct temperature phases—the one before ovulation and the one after ovulation.

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

Triphasic chart

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

With a triphasic chart, there are three temperature shifts. For a chart to be truly triphasic, this third temperature shift should occur at least seven days after ovulation.

Look at the example chart above. Do you see there is a third temperature shift starting on Day 25? This shift occurred ten days after ovulation. However, even if it started a little earlier—say just seven days after ovulation—it could still indicate that the chart showed a triphasic pattern.

Triphasic temperature spikes can indicate pregnancy but that's not always the case. And you can be pregnant without seeing a triphasic pattern.

Choosing a BBT Chart

The first step to charting your basal body temperature is getting a chart to record your temperature. You can find sample charts in some fertility books, such as Take Charge of Your Fertility—a book considered by many to be the go-to resource for basal body temperature charting guidance.

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

You can also make your own graph. If you make your own, you will want to plot the temperature along the vertical, allowing one-tenth of a degree for each square. Along the horizontal, you will have the days of your cycle.

Most people prefer using a digital tool because you can log a ton of information and reduce the chance of human error. Most ovulation software will automatically indicate when ovulation likely occurred. If you try plotting the temperature yourself, you might worry about making a mistake.

How to Measure BBT

Once you have something to record your temperature on, it's time to start taking your basal body temperature. You will need a thermometer—any store bought option will work. There are a few important guidelines to follow while taking your BBT.

Timing

Take your temperature at the same time (plus or minus no more than 30 minutes) every morning. For example, if you take it at 7:30 a.m. every weekday, don't take it earlier than 7 a.m. or later than 8 a.m. on other days.

Movement

Avoid getting up, sitting up, walking around, or going to the bathroom before taking your temperature. The minute after you wake up, you need to pop the thermometer in your mouth.

Sleep

Make sure you have had at least three to four straight hours of sleep before taking your temperature in the morning. If you stayed up all night, or you woke up and walked around at night repeatedly, it will throw off your results.

Reliability of Basal Body Temperature Charts

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

In their informal analysis, they considered a triphasic pattern to be a second, significant upward shift in temperature of at least 0.3 F, occurring at least 7 days after ovulation.

In practice, there’s no definitive definition of a triphasic chart. People comparing and sharing charts may disagree on whether a certain pattern could be considered triphasic. The definition here is just for analysis.

So, based on this data, a chart indicating a triphasic pattern is three times more likely to belong to someone who is pregnant.

There is a very important fact to point out here, in case you missed it: While 12% of pregnancy charts had the triphasic pattern, 88% did not.

Or, to put it another way, if you looked at the pregnant BBT charts of 100 people, just 12 of them would show the triphasic pattern. If you don’t see the pattern, however, this doesn’t mean you’re not pregnant. It's also important to remember that having the triphasic chart doesn’t always mean you are pregnant.

Pregnancy and Basal Body Temperature Charts

Basal body temperature can reveal interesting insights on pregnancy. If you are not pregnant, progesterone levels drop, leading to a decrease in basal body temperature. However, pregnant people typically have increased levels of progesterone, to help support the pregnancy. Therefore, a sustained increase in BBT could signal pregnancy. However, everyone is different and some pregnant people may not experience rising basal body temperature. In addition to the triphasic pattern, here are three more ways a BBT chart can indicate a pregnancy or the possibility of pregnancy.

Sex on Most Fertile Days

A BBT chart can help you determine if you have had sex on your most fertile days. Although your basal body temperature can’t predict ovulation, you can determine if and when you ovulated a few days after it happened in a BBT chart.

This means you may not know if you had sex on the “right days” until after ovulation occurs. But you can look back on your chart and determine this. You are most likely to conceive if you had sex on the two days preceding ovulation.

Implantation Dip

An implantation dip is a one-day drop in temperature about a week after ovulation. The majority of the time, an implantation dip is nothing more than a mid-cycle dip in temperature and does not indicate pregnancy. It’s debatable whether or not this is a possible sign of early pregnancy.

Long Luteal Phase

The most reliable way to detect pregnancy on a BBT chart takes patience. The old-fashioned method of doing this 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 if the length of their menstrual cycle does vary. For example, a cycle length may range from 30 and 35 days, but the luteal phase is consistently 12 or 13 days long.

If you see that your luteal phase has gone at least one day past the usual length, you might be pregnant. If it goes two days past the longest luteal phase you’ve ever had, the likelihood of being pregnant is even higher. This is a good time to take a pregnancy test.

If you reach 18 days past ovulation and you still don’t have your period, chances are very good that you are pregnant. Not many people can wait that long without taking a pregnancy test. Still, it is the strongest early sign of pregnancy detectable with a BBT chart.