BBT Charting for Beginners

What is Basal Body Temperature?

Basal body temperature (BBT) is your body's temperature at complete rest—the lowest temperature your body reaches during sleep. It's different from your regular body temperature because even small activities (like getting out of bed) can raise it.

For fertility tracking, BBT is valuable because it responds predictably to hormones:

• Before ovulation (follicular phase): Temperatures typically range from 96.0-97.5°F (35.5-36.4°C)
• After ovulation (luteal phase): Progesterone causes temps to rise 0.2-0.5°F, typically ranging 97.6-98.6°F (36.4-37°C)

This temperature shift confirms that ovulation has occurred. When you see the sustained rise, you know your body released an egg.

How BBT Tracking Works

The Biphasic Pattern

A typical ovulatory cycle shows two distinct temperature phases:

1. Follicular phase (lower temps): From period start until ovulation
2. Luteal phase (higher temps): From ovulation until next period

The shift between these phases confirms ovulation. You're looking for temps that rise and stay elevated for at least 3 consecutive days.

What BBT Can and Can't Tell You

Getting Started with BBT Charting

What You Need

• BBT thermometer: Must read to 1/100th of a degree (e.g., 97.68°F) for accuracy
• Chart or app: To record daily temperatures
• Consistency: Same time each day, immediately upon waking

Step-by-Step Instructions

1. Get a BBT thermometer. Regular fever thermometers aren't precise enough. BBT thermometers show two decimal places.
2. Keep it on your nightstand. You need to take your temperature before doing anything—even sitting up or going to the bathroom.
3. Take your temp at the same time daily. Set an alarm if needed. Temps can vary by 0.1°F per hour of sleep difference.
4. Take your temp after at least 3-4 hours of sleep. Ideally after your longest sleep stretch.
5. Record immediately. Log the temperature in your app or chart before you forget.
6. Note any factors that might affect readings. Poor sleep, alcohol, illness, travel, etc.
7. Start on cycle day 1. Begin a new chart on the first day of your period (first day of real flow, not spotting).

Oral vs. Vaginal Temping

Most people take BBT orally (under the tongue), but vaginal temps can be more consistent for some:

Whichever you choose, be consistent. Don't switch mid-cycle.

Reading Your BBT Chart

Identifying the Temperature Shift

Look for these signs that ovulation occurred:

1. Coverline: Draw a horizontal line 0.1°F above your highest of the last 6 low temps. Your post-ovulation temps should stay above this line.
2. Three-day rule: Ovulation is confirmed when you have 3 consecutive temps higher than the previous 6. This is called the "3 over 6" rule.
3. Sustained elevation: Temps should stay elevated (with minor fluctuations) until your period arrives.

The Implantation Dip

Some charts show a one-day temperature dip around 7-10 DPO, followed by a rise. This is sometimes called an "implantation dip" and may indicate pregnancy—but it's not reliable:

• Only ~25% of pregnancy charts show an implantation dip
• Non-pregnancy charts can also show random dips
• Don't read too much into a single day's reading

Triphasic Pattern

A triphasic chart shows three distinct temperature levels: pre-ovulation (low), post-ovulation (elevated), and a second rise around 7-10 DPO (higher still). This pattern is more common in pregnancy charts but isn't definitive.

Common BBT Patterns

Normal Ovulatory Chart

• Clear low temps before ovulation
• Obvious shift of 0.2°F+ that stays elevated
• Luteal phase of 10-16 days before period or positive test

Slow Rise

• Temperature rises gradually over 3-4 days rather than sharply
• Still indicates ovulation—just a different pattern
• Common and usually normal

Fallback Rise

• Temp rises, dips back down for a day, then rises again
• The initial rise day is usually ovulation day
• Hormones can fluctuate—one day doesn't negate the pattern

No Clear Shift (Anovulatory)

• Temps stay relatively flat throughout the cycle
• No distinct high and low phase
• Suggests ovulation may not have occurred
• Occasional anovulatory cycles are normal; frequent ones warrant a doctor visit

Short Luteal Phase

• Temps rise but fall after only 8-9 days
• May indicate luteal phase defect
• Worth discussing with your doctor if consistent

Tips for Accurate BBT Readings

Do:

• Take your temp at the same time every day (within 30 minutes)
• Take it immediately upon waking—before sitting up, talking, or checking your phone
• Get at least 3-4 consecutive hours of sleep before temping
• Use the same thermometer all cycle
• Note factors that might affect readings (illness, alcohol, poor sleep)
• Chart for at least 3 cycles to see your pattern

Don't:

• Stress about one "off" reading—look at the overall pattern
• Compare your temps to others—everyone's baseline is different
• Switch between oral and vaginal mid-cycle
• Take your temp after getting up to use the bathroom
• Rely solely on BBT for timing sex (it confirms ovulation after the fact)

Factors That Can Affect BBT

Best BBT Thermometers