Types of Thermal Shift
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14 January 2017

Types of Thermal Shift

Types of Thermal Shift
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Written by Dr. Surveen Ghumman
The BBT chart and fertility calculator are a woman best friends. Fertility and ovulation are now easy to identify with the thermal shift being a major indicator

The BBT chart and fertility calculator are a woman best friends. Fertility and ovulation are now easy to identify with the thermal shift being a major indicator

In an ideal world, all women would have straightforward and easy-to-read BBT charts that show their ovulation patterns very clearly, so everyone could conveniently know exactly when they are fertile. Fertility calculators would have been a thing of the bygone. Unfortunately, this is not the case. Each woman is different and so is her BBT chart. In this section, we’ll go over the most common types of BBT charting patterns, a.k.a. thermal shifts.

1. Biphasic Curve or Standard Thermal Shift Pattern: While there’s no such thing as an “abnormal” chart, some charts do look more typical than others. The most common type is the biphasic curve or standard thermal shift pattern. In the biphasic curve, there are 2 phases: a clear set of low temperatures, followed by a distinct shift of at least two-tenths (0.2) degrees Fahrenheit, then another range of higher temperatures that last until the end of the cycle. This type of chart is the easiest to read and draw a cover line for. You can begin to get a sense of whether or not your chart is biphasic once your temperatures have stayed elevated for at least 3 to 4 days after ovulation, but it’s best to wait until the end of your menstrual cycle to determine.

Though most women will experience the same kind of patterns within their own cycles, it’s also normal to occasionally see some variation. The biphasic curve is the most common, but there are other types of patterns to keep in mind as well.

2. Slow Rise Pattern: Another BBT chart type is the monophasic or slow rise pattern. On monophasic charts, there’s no distinct jump in temperature but instead, a gradual rise.

Ovulation, Fertility Calculator, BBT Chart

For example, temperatures may increase slightly by one-tenth (0.1) of a degree Fahrenheit each day. Note that the Fertility Awareness Method rules for drawing cover lines don’t work for monophasic charts.

3. Triphasic Curve or Stair-Step Rise Pattern: The triphasic curve or stair-step rise pattern is a chart in which the temperatures rise twice after ovulation, creating 3 phases on the chart. The initial rise may last for a few days, then the temperatures rise again for the second time, typically 7 to 10 days after ovulation. 

Some women wonder if the second rise indicates pregnancy, but there’s no scientific evidence that supports that triphasic curves reliably mean pregnancy. The best way to figure out if you’re pregnant is to watch for elevated BBT beyond 14 to 15 days after ovulation without a period.

4. Fall-Then-Rise Pattern: In the fall-then-rise pattern, a distinct rise is observed, temperatures drop or fall slightly again, and then immediately rise again for the rest of the cycle. Keep in mind that there are some circumstances where it might be tough to distinguish fall-then-rise pattern from a chart with a random, outlier temperature. In these cases, use your other non-temperature tracking methods (cervical mucus or cervical position) or luteal phase length to determine whether or not the chart is demonstrating a fall-then-rise pattern.

5. Zig-Zag Pattern: As the name implies, temperatures may rise in a pattern that resembles a "zig-zag." In this pattern, the temperatures may rise, fall slightly, then rise again for a few days. 

6. Ovulation Dip (Dip Before Rise): Some women observe a slight temperature dip right before ovulation and its subsequent temperature rise. This is known as an “ovulation dip.” For those who are lucky and experience this dip consistently throughout their cycles, it can serve as a (very) helpful indication that ovulation is coming soon. Keep in mind that just because you experience a dip, it doesn’t necessarily mean that ovulation is coming. In other words, it’s possible that you can experience a dip but it’s actually not followed by a sustained rise. An individual dip can’t tell you anything about ovulation or your fertility so make sure that you always confirm that ovulation occurred by looking for a subsequent temperature rise.

7. Ambiguous Thermal Shift: Unfortunately, not all thermal shifts are clear or obvious. With this kind of pattern, it can be tough to pinpoint when exactly ovulation occurred. Shifts can be ambiguous for a number of reasons: temperatures rise slowly; some dips occur, or data could be missing or conflicting.

BBt Chart, Fertility, Ovulation

In these cases, it’s helpful to also track other fertility signs, such as cervical mucus or cervical position. Trust us, we understand how frustrating this can be, but the best thing to do is assume the latest possible date for ovulation. Also, Ela will be depending on other factors to help you out as well. 

8. No Thermal Shift: Sometimes, women record BBT charts with no clear patterns of high and low temperatures even when they are doing all the “DOs” in our “DOs and DON’Ts of BBT Charting.” If this is you, then there are few things that could be going on.

One case being that there is a small number of women whose bodies don’t respond to progesterone, despite having actually ovulated. If in fact, this situation does apply to you, then you always have the option to track your fertility by charting cervical mucus and/or cervical position, too.

It’s also possible to have an anovulatory cycle, which wouldn’t have a thermal shift since there’s no heat-producing progesterone released from the corpus luteum (because you didn’t ovulate). Anovulatory cycles can be temporarily onset by factors, like illness, stress, or a follicular ovarian cyst. However, multiple anovulatory cycles could be indicative of medical conditions such as Polycystic Ovarian Syndrome (PCOS). If you suspect that this may be the case, please reach out to your doctor to discuss it further.

Lastly, menopause could also be a possibility. As women approach menopause, they stop ovulating as much as they used to.

Note: When it comes to BBT charts, there can be all sorts of pattern variation. The patterns we described above might be a little bit tricky to read at first, but once you get familiar, they’ll become way easier to interpret. 

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2 Comments

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Antra Shekhar
Sept. 19, 2017, 5:12 p.m.
BBT or what we call a basal body thermometer is a specially designed, large-scale, easy-to-read thermometer that points from 96 to 100 degrees only. Most women get 96 to 98 degrees before ovulation and 97 to 99 degrees after ovulation. With a BBT, one can easily track their ovulation period.
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Ava Williams
Sept. 19, 2017, 5:20 p.m.
Its quite easy to track ur ovulation with a fertility calculator too.. ...sometimes we need to know the average length of your menstrual cycle so that we can get to know our ovulation. There we can calculate the fertile window thru a fertility calculator..just put in ur cycle length like 28 days ...
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