What Triphasic Charts Mean for Early Pregnancy

Recognizing the Triphasic Pattern on a Basal Body Temperature Chart

BBT (Basal body temperature) chart with triphasic shift illustrated
Rachel Gurevich

A triphasic chart is a basal body temperature (BBT) chart with three distinct temperature rises. (More on this below.) This pattern is thought to be a possible sign of pregnancy, and 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 a sudden intense awareness of other possible early pregnancy symptoms.

But how much of this excitement is justified?

How can you spot a triphasic chart? What causes this pattern to occur, theoretically speaking?

Is it a reliable pregnancy sign?

What Is a Triphasic BBT Chart?

First, it would be helpful to define biphasic. Every basal body temp 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 temperatures.

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.

Now, with a triphasic chart, there are three temperature shifts.

Tri means three, which you probably already know from words like tricycle. 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—we could say the chart showed a triphasic pattern.

Is a Triphasic Chart a Reliable Early Pregnancy Sign?

FertilityFriend.com, 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 was by no means a 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 real definitive definition of a triphasic chart. Women comparing and sharing charts may disagree whether a certain pattern could be considered triphasic or not. The definition here is just for the purposes of analysis.)

After analyzing almost 150,000 BBT charts, they found that 12 percent of all pregnancy charts showed a triphasic pattern. When looking at non-pregnancy charts, they found that only 5 percent 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 them:

  • While 12 percent of pregnancy charts had the triphasic pattern, 88 percent did not.

Or, to put it another way, if you looked at the pregnant BBT charts of 100 women, just one or two of them would show the triphasic pattern. If you don’t see the pattern, this doesn’t mean you’re not pregnant!

Also important, having the triphasic chart doesn’t mean you are pregnant. As the statitics above stated, 5 out of every 100 charts of non-pregnant women still showed a triphasic pattern. Some women regularly get triphasic pattern charts, and it doesn't indicate pregnancy.

Causes of the Triphasic Pattern

What causes that third temperature shift?

For a non-pregnancy chart, a triphasic chart could be caused by a difference in your bedroom temperature, a slight illness (not enough to cause a fever but maybe a slight temp rise), or your hormones getting overly excited about not much.

What if you are pregnant? In that case, the triphasic pattern could be caused by further increases in the hormone progesterone.

It is the hormone progesterone that 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.

Should You Take an Early Pregnancy Test If You See a Triphasic Pattern?

Any excuse to take an early pregnancy test, am I right? And maybe this is the month you finally see an early 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.

You may get a BFN (big fat negative) even if you are pregnant, and that would be a bummer.

Consider holding off on testing until either your period is late or you show 16 high temperatures on your chart. Sixteen high temperatures is the best sign of pregnancy you can find on a BBT chart.

A Word From Verywell

Basal body temperature charting is an excellent way to get to know your menstrual cycle better, detect ovulation day, and learn what your most fertile days of the month are. You may be tempted to look for signs of pregnancy on your chart, like the triphasic pattern, but the most reliable sign of pregnancy on a BBT chart is that your luteal phase passed 16 days. In other words, your period is a few days late. While you're more likely to see a triphasic pattern on your chart if you're pregnant, seeing one doesn't mean you for sure are pregnant... and not seeing one doesn't mean you aren't.

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