What Triphasic Basal Body Temperature Charts Mean for Early Pregnancy
Does a triphasic chart mean you'll get a positive pregnancy test soon? Maybe! Here's how to recognize one in your BBT chart.
Medically reviewed by Alyssa Dweck, MS, MD, FACOG
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. Because of this, these kinds of charts are deeply coveted across the fertility charting community.
Getting 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.
Seeing triphasic temperature spikes can indicate pregnancy but that's not always the case. And you can be pregnant without seeing a triphasic pattern.
Learn more about triphasic basal body temperature, including what causes this pattern to occur and if it's a reliable pregnancy sign.
Parents / Rachel Gurevich
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
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.
Ovulation is indicated on a BBT chart by a distinct and sustained upward shift in body temperature.
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
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.
Related: The Best Basal Thermometers to Help You Kick-Start Your Family Planning Journey
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.)
After analyzing almost 150,000 BBT charts, researchers found that 12% of all pregnancy charts showed a triphasic pattern. When looking at non-pregnancy charts, they found that only 4.5% of charts showed a triphasic pattern.
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!
Also important, having the triphasic chart doesn’t always mean you are pregnant.
As the statistics above stated, roughly 5 out of every 100 charts (4.5%) of non-pregnant people still showed a triphasic pattern. Some people may regularly get triphasic pattern charts, and it doesn't always indicate pregnancy.
You may be tempted to look for signs of pregnancy on your chart, like the triphasic pattern. However, the most reliable sign of pregnancy on a BBT chart is that your luteal phase (the time after ovulation before your period) passed 16 days.
Related: All About Timing Sex for Pregnancy
Causes of Triphasic Basal Body Temperature Charts
For a non-pregnancy chart, a triphasic chart could be caused by the following:
A difference in your bedroom temperature
Your level of hydration
Improper use of the test (like taking it after performing an activity in the morning)
A slight illness (not enough to cause a fever but maybe a slight temperature rise)
Your hormone levels changing for a non-pregnancy-related reason
What if you are pregnant? In that case, the triphasic pattern could be caused by further increases in the hormone progesterone.
The hormone progesterone causes the original shift up at the time of ovulation.
Progesterone triggers your uterine lining to prepare for the implantation of an embryo, suppresses ovulation (which is why you can’t get pregnant when you’re already pregnant), and prevents the endometrium from shedding when there may be an embryo or baby in there.
The theory is that implantation of an embryo triggers increased production of the hormone progesterone. That sudden boost may cause another shift up in temperature.
Related: 6 Signs of Fertility To Pay Attention To
Basal Body Temperature Charts Pregnant vs Not Pregnant
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.
When to Take a Pregnancy Test
Any excuse to take an early pregnancy test, right? And maybe this is the month you finally see an early BFP (Big Fat Positive)! Or maybe not.
There are many good reasons not to take an early pregnancy test. You might think that a triphasic pattern is a good reason to go ahead and test before your period is late.
Keep in mind, however, that pregnancy tests look for the pregnancy hormone hCG—and not progesterone. Even if your progesterone levels are slightly higher, it doesn’t mean that your pregnancy hormones are higher. It may take several more days for the hCG levels to build up enough to be detectable by a pregnancy test.
Consider holding off on taking a pregnancy test until either your period is late or you show 16 days of high temperatures on your basal body temperature chart for the most accurate results.
Related: Why hCG Doubling Times Are Important in Early Pregnancy
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