How do I measure basal body temperature (BBT) for fertility tracking?

How to Measure Basal Body Temperature for Fertility Tracking

Measure your basal body temperature (BBT) immediately upon waking, before any physical activity, using a digital thermometer placed under your tongue, and record the temperature at the same time each morning throughout your menstrual cycle to detect the biphasic temperature shift that indicates ovulation has occurred.

Measurement Technique

Timing and Method

• Take your temperature immediately upon waking, before getting out of bed, talking, eating, or drinking 1
• Measure at the same time each morning to minimize circadian rhythm variations that can affect accuracy 2
• Use an oral digital thermometer placed under the tongue for consistent readings 2
• Record the temperature daily throughout your entire menstrual cycle 1

What to Look For

• The biphasic pattern: BBT remains lower during the follicular (pre-ovulation) phase and rises 0.2-0.5°C after ovulation occurs 2, 3
• The temperature shift typically occurs within a range of 5 days before to 4 days after ovulation, making it more useful for confirming ovulation has occurred rather than predicting it in advance 3
• A sustained temperature elevation for at least 3 consecutive days suggests ovulation has occurred and the luteal phase has begun 1, 4

Important Limitations and Caveats

Reliability Concerns

• BBT does not reliably predict ovulation in advance—it primarily confirms ovulation has already occurred 1
• Only 10 of 66 normal cycles showed the classic "temperature dip" before ovulation, and timing varied widely 3
• BBT gave incorrect information about ovulatory status in approximately 7% of cycles 3
• Single daily measurements may miss the temperature shift due to circadian rhythm variations 2

Conditions That Interfere with BBT Accuracy

Avoid BBT methods if you have:

• Acute febrile illness (fever from infection)—delay using temperature-based methods until the illness resolves 1
• Chronic diseases causing elevated temperature—temperature-based methods are not appropriate for women with chronically elevated temperatures 1
• Irregular menstrual cycles—this makes BBT patterns difficult to interpret 1

Use caution if you have:

• Medications affecting temperature: lithium, tricyclic antidepressants, anti-anxiety medications, certain antibiotics, or anti-inflammatory drugs may alter BBT patterns 1
• Postpartum status: BBT methods require at least 4 weeks postpartum in non-breastfeeding women and are more complex in breastfeeding women 1

Alternative and Complementary Approaches

Modern Wearable Devices

• Wrist skin temperature monitors worn continuously during sleep (such as fertility tracking bracelets) are more sensitive than traditional BBT for detecting ovulation (62% vs 23% sensitivity) 2
• These devices capture the greater temperature increase in the postovulatory phase (0.50°C vs 0.20°C for oral BBT) and provide continuous data rather than single point measurements 2, 5
• However, wearable devices have lower specificity (26% vs 70%), meaning more false positives 2

Combined Fertility Awareness Methods

• Symptothermal methods combine BBT with cervical mucus observation for improved accuracy 1
• Cervical secretions are not affected by fever or temperature-related conditions, unlike BBT 1
• Using multiple fertility signs together provides more reliable information than BBT alone 1

Common Pitfalls to Avoid

• Don't rely on BBT alone for pregnancy prevention—typical-use failure rates for fertility awareness methods are approximately 24% 1
• Don't use BBT during acute illness—elevated temperatures from infection make interpretation impossible 1
• Don't expect BBT to predict ovulation before it happens—the temperature rise occurs after ovulation, making it useful primarily for confirming ovulation retrospectively 1, 3
• Don't measure temperature after any physical activity—even minimal movement can affect the reading 2