What is Basal Body Temperature?
Your Basal Body Temperature (BBT) is your body's lowest resting temperature — the temperature your body settles into during deep, restful sleep. This subtle measurement, typically taken first thing in the morning before any physical activity, provides a window into your hormonal rhythms and, most importantly, confirms when ovulation has occurred.
For women trying to conceive, BBT charting is one of the most trusted and time-tested methods of fertility awareness. While it won't predict ovulation in advance (like ovulation predictor kits do), it provides confirmation that ovulation has happened — invaluable information for understanding your unique cycle patterns.
💡 Key Insight
After ovulation, your body releases progesterone, which causes a slight but measurable rise in your basal temperature. This "thermal shift" typically ranges from 0.4°F to 0.8°F (0.2°C to 0.5°C) and remains elevated until your next period begins — or stays high if pregnancy has occurred.
Think of BBT charting as learning your body's unique language. Over 2-3 cycles of consistent tracking, you'll begin to see patterns emerge that help you anticipate your fertile window with increasing accuracy. Many women find this practice deeply empowering — it transforms the abstract concept of ovulation into something tangible and observable.
How to Measure Your BBT Correctly
Accuracy is everything when it comes to BBT charting. Because we're measuring tiny temperature fluctuations — sometimes just tenths of a degree — consistency in your measuring technique is crucial. Here's how to get the most reliable readings:
Take Your Temperature at the Same Time Every Day
Set an alarm for the same time each morning — even on weekends. Your BBT naturally fluctuates throughout the day, so consistency is key. Most fertility experts recommend measuring between 6:00 AM and 8:00 AM, but the exact time matters less than taking it at the same time daily. Even a 30-minute variation can affect your reading.
Measure Immediately Upon Waking
This is perhaps the most important rule: take your temperature before you do anything else. Don't sit up, don't check your phone, don't even talk. Any physical activity — even getting up to use the bathroom — can raise your body temperature and skew your reading. Keep your thermometer on your nightstand within arm's reach.
Get at Least 3-4 Hours of Uninterrupted Sleep
For an accurate basal reading, your body needs time to reach its true resting state. If you've been up nursing a baby, dealing with insomnia, or had a restless night, note it on your chart. These disrupted readings can still be useful data points, but you'll want to identify them as potentially less reliable.
Use the Same Method Consistently
You can take your BBT orally, vaginally, or rectally — but pick one method and stick with it for the entire cycle. Oral measurement is most common and convenient, but vaginal or rectal readings tend to be more stable and less affected by mouth breathing during sleep. If using oral, place the thermometer under your tongue in the same spot each time.
Record Your Temperature Immediately
Write down or log your temperature right away — you'll forget the exact number surprisingly quickly! Many BBT thermometers have memory recall features, but it's best practice to record immediately. Use a dedicated app, a paper chart, or a notebook kept by your bed.
Factors That Can Affect Your BBT Reading
- Illness or fever — Even a mild cold can elevate temperatures
- Alcohol consumption — Drinking the night before often raises morning temps
- Poor or disrupted sleep — Less than 3 consecutive hours affects readings
- Travel or time zone changes — Jet lag disrupts your body's rhythms
- Certain medications — Including antihistamines, thyroid meds, and some pain relievers
- Sleeping with an electric blanket — Can artificially elevate your temperature
- Mouth breathing during sleep — Affects oral temperature readings
When any of these factors are present, note them on your chart. The reading is still valuable data, but should be interpreted with context.
Recognizing the Thermal Shift: Confirming Ovulation
The moment you've been watching for: the thermal shift. This is the telltale sign that ovulation has occurred. After the egg is released from your ovary, the empty follicle transforms into a structure called the corpus luteum, which produces progesterone. This hormone is thermogenic — meaning it raises your body temperature.
❄️ Pre-Ovulation (Follicular Phase)
Your temperatures are generally lower, typically ranging from 97.0°F to 97.6°F (36.1°C to 36.4°C). You may notice some day-to-day variation, but overall, these "pre-shift" temperatures form a relatively stable baseline.
🔥 Post-Ovulation (Luteal Phase)
After ovulation, your temperatures rise by approximately 0.4°F to 0.8°F (0.2°C to 0.5°C), typically reaching 97.6°F to 98.6°F (36.4°C to 37°C). These elevated temps remain high for 10-16 days until your next period — or longer if you're pregnant.
How to Identify the Shift
Fertility awareness experts use the "Three over Six" rule (also called the "coverline method") to confirm ovulation:
The Three-Over-Six Rule
- Step 1: Look at your most recent 6 temperatures before the suspected shift
- Step 2: Draw a "coverline" 0.1°F above the highest of those 6 temperatures
- Step 3: Confirm ovulation when you see 3 consecutive temperatures above the coverline
Once you have 3 temps above the coverline, ovulation is confirmed to have occurred on or near the day before the first elevated temperature.
Example BBT Chart Pattern
Blue bars: Pre-ovulation temps | Orange: Ovulation day | Red bars: Post-ovulation temps (above coverline)
What About the "Pre-Ovulation Dip"?
Some women notice a slight temperature dip just before ovulation — this is caused by a surge in estrogen that occurs right before the LH surge triggers egg release. While not every woman experiences this dip, when it does occur, it can serve as an early warning that ovulation is imminent. Look for it on Day 12-14 of a typical 28-day cycle.
🎯 What a Sustained High Temperature Means
If your temperatures remain elevated for 18 or more days after ovulation, this is a strong early sign of pregnancy. The sustained progesterone production from a developing pregnancy keeps temperatures high. This is often one of the first physical indicators — sometimes detectable even before a missed period or positive pregnancy test.
Best Thermometers for BBT Tracking
Not all thermometers are created equal when it comes to fertility tracking. A regular fever thermometer typically only measures to one decimal place (e.g., 98.6°F), which isn't precise enough to detect the subtle 0.4°F shift after ovulation. For accurate BBT charting, you need a basal body thermometer that measures to two decimal places (e.g., 98.63°F).
Basic Digital BBT Thermometer
Best for beginners
- Measures to 1/100th degree
- Memory recall for last reading
- Affordable ($10-20)
- Manual charting required
Popular options: Easy@Home, iProven, Femometer Basic
Smart/Connected BBT Thermometer
Best for accuracy + convenience
- Syncs automatically to app
- Auto-generates charts
- Predicts ovulation based on patterns
- Backlit display for dark mornings
Popular options: Tempdrop, Femometer Vinca II, Ava Bracelet
Wearable BBT Sensor
Best for irregular sleepers
- Takes readings while you sleep
- No wake-up time requirement
- Great for shift workers & new parents
- Higher price point ($150-300)
Popular options: Tempdrop Armband, Ava Bracelet, Oura Ring
Glass Basal Thermometer
Traditional/Old-school option
- No batteries required
- Very precise readings
- Takes 5+ minutes to read
- Fragile / difficult to read in low light
Note: Glass thermometers are becoming less common and may be harder to find.
✓ Features to Look For in a BBT Thermometer
- Measures to 2 decimal places (1/100°)
- Memory recall function
- Backlit display
- Quiet beep (or silent mode)
- Flexible tip (for comfort)
- Fast reading time (under 60 seconds)
Combining BBT with Other Fertility Signs
While BBT charting is powerful on its own, it becomes even more reliable when combined with other fertility awareness methods. This multi-sign approach — often called the Sympto-Thermal Method — gives you a more complete picture of your fertility.
Cervical Mucus Monitoring
As estrogen rises before ovulation, cervical mucus becomes stretchy, clear, and egg-white consistency (EWCM). This is a pre-ovulation sign that warns you fertility is approaching — complementing BBT which confirms ovulation after it happens.
Ovulation Predictor Kits (OPKs)
OPKs detect the LH surge that occurs 24-48 hours before ovulation. Use them starting around day 10 of your cycle. A positive OPK combined with fertile cervical mucus and followed by a BBT rise gives you high confidence in your ovulation timing.
Cervical Position
During your fertile window, the cervix moves higher, becomes softer, and opens slightly. After ovulation, it drops lower, firms up, and closes. This can be felt by gently inserting a clean finger into the vagina.
Ready to Start Tracking?
Use our free Ovulation Calculator to predict your fertile window based on your cycle data. Combine it with BBT charting to confirm your predictions and build a complete picture of your fertility.
Use Our Free Ovulation CalculatorFrequently Asked Questions
How many cycles should I chart before I can predict ovulation?
My temperatures are all over the place. What am I doing wrong?
Can BBT charting tell me when I'm about to ovulate?
What if I work night shifts or have an irregular sleep schedule?
Final Thoughts: Your Body Is Speaking — Learn to Listen
Basal Body Temperature charting is more than just a fertility tracking method — it's a practice of deep body awareness. In a world where we're often disconnected from our physical selves, taking a few seconds each morning to measure your temperature is an act of tuning in, of paying attention to the subtle rhythms that govern your reproductive health.
We understand that the journey to conception can be emotionally complex. There are months when the thermal shift arrives right on schedule, and months when your chart looks nothing like the textbook examples. That's okay. Your body is unique, and your patterns are your own. Trust the process, be patient with yourself, and remember that each cycle is an opportunity to learn something new.
Whether you're just starting out or you've been charting for months, we're here to support you with the tools and knowledge you need. You've got this. 💗