A normal BBT chart has two distinct temperature levels: lower temperatures in the first half of your cycle and higher temperatures in the second half, with a clear shift between them. This two-level pattern, called a biphasic chart, is the hallmark of a cycle where ovulation occurred. The shift itself is small, typically between 0.4°F and 1°F, but it’s visible when you chart daily readings over time.
The Two Phases of a Normal Chart
Before ovulation, your temperatures cluster in a lower range. This is your follicular phase. The exact numbers vary from person to person, but many people see readings somewhere around 97.0 to 97.5°F during this stretch. Temperatures may bounce around a bit day to day, but they stay within a general band.
After ovulation, progesterone floods your system. This hormone acts on the temperature-regulating area of your brain, nudging your baseline body temperature upward. Your chart shifts to a higher range, typically 0.5° to 1°F above where it was before. These elevated temperatures hold steady for the rest of your cycle until your period arrives. The result is a chart that looks like a set of stairs: a lower level, then a step up that stays up.
The shift between these two phases marks ovulation. Some people notice a slight dip in temperature just before the rise, but the key feature is the sustained climb afterward. You can confirm ovulation has occurred when you see higher temperatures for at least three consecutive days compared to the six days before the shift.
What Each Part Should Look Like
The Pre-Ovulation Phase
This stretch covers the first portion of your cycle, starting from the first day of your period. Temperatures here tend to be relatively stable, though some day-to-day variation is normal. A few erratic readings can happen if you slept poorly, drank alcohol the night before, were stressed, took pain medication, or were fighting off an illness. These are “disturbed” temperatures, and it helps to note them on your chart so you don’t mistake them for a real pattern.
The Temperature Shift
The shift itself can look different from chart to chart. Some people see a dramatic jump over one or two days. Others see a more gradual rise over three days. Both are normal. What matters is that the temperatures clearly settle into a higher range and stay there. The rise can be as small as 0.4°F or as large as 1°F, so you need a thermometer that reads to two decimal places (like 97.62°F) to catch it reliably.
The Post-Ovulation Phase
After the shift, temperatures should remain elevated for roughly 10 to 16 days. This is your luteal phase. A luteal phase shorter than 10 days can signal that progesterone levels are dropping too quickly, which may make it harder for a fertilized egg to implant. If your temperatures consistently fall back down before 10 days past your shift, that’s worth bringing up with a healthcare provider.
Right before your period starts, temperatures typically drop back down to the lower range. This drop signals that progesterone has fallen and your next cycle is beginning.
The Implantation Dip
Some charts show a one-day temperature dip about 7 to 8 days after ovulation, followed by a return to the higher range. This is sometimes called an implantation dip, and it gets a lot of attention in fertility communities. A large analysis by the fertility tracking app Fertility Friend found that this dip appeared in 23 percent of charts that resulted in pregnancy, compared to 11 percent of charts that didn’t. So while it’s slightly more common in conception cycles, it’s not a reliable pregnancy sign on its own.
The Triphasic Pattern
A triphasic chart shows a third temperature increase roughly 7 to 10 days after ovulation. Instead of just two levels (pre-ovulation and post-ovulation), there’s a noticeable step up to a third, even higher level. This happens because of a secondary rise in progesterone as a pregnancy establishes itself. A triphasic pattern is more strongly associated with pregnancy than a standard biphasic chart, though it’s not a guarantee. Some non-pregnant cycles show a triphasic pattern too.
What an Anovulatory Chart Looks Like
If ovulation didn’t happen, your chart won’t show that clear two-level pattern. Instead, temperatures stay in roughly the same range throughout your entire cycle, with no sustained rise. This flat, single-level pattern is called monophasic. It means your body didn’t produce the progesterone surge that causes the temperature shift.
An occasional anovulatory cycle is normal, especially during times of high stress, illness, or significant weight changes. But if your charts consistently look monophasic cycle after cycle, it could point to a hormonal issue. People with polycystic ovary syndrome (PCOS), for example, often show disorganized, erratic temperature curves with no clear shift. Their charts may also have unusually long first phases, reflecting delayed or absent ovulation.
Getting Accurate Readings
The temperature changes you’re looking for are tiny, so your method matters. Use a BBT-specific thermometer that reads to two decimal places. Take your temperature at the same time each morning, before you get out of bed, talk, eat, or drink anything. Your true basal temperature is actually lowest around 4 a.m., but since most people don’t want to set an alarm for that, a consistent waking time works as a reliable substitute.
Several things can throw off a single day’s reading: alcohol the night before, a disrupted night of sleep, travel across time zones, illness, stress, and pain medications like ibuprofen or acetaminophen. When this happens, mark that temperature as potentially unreliable on your chart. One off reading won’t ruin your overall pattern, but if you don’t flag it, you might misinterpret your shift day.
Never switch thermometers or change where you take your temperature (oral versus underarm, for example) in the middle of a cycle. Different thermometers and different body sites can give slightly different baseline readings, which makes it harder to spot the small shift you’re tracking.
If you have irregular wake times or disrupted sleep, wearable temperature sensors that track continuously overnight can give more consistent data than a single morning reading. These devices filter out the noise of inconsistent sleep schedules and calculate your baseline temperature from hours of data rather than one snapshot.
How Many Cycles Before You See a Pattern
One chart tells you about one cycle. To understand your own pattern, you need at least two to three cycles of data. Over time, you’ll learn where your personal pre-ovulation range sits, how dramatic your shift tends to be, and roughly which cycle day you typically ovulate. Some people shift on day 14 like the textbook says, but plenty of healthy cycles shift earlier or later. Your consistent pattern across multiple charts is what matters, not matching a generic template.