Why Is My BBT High in the Follicular Phase?

Noticing a higher-than-usual basal body temperature (BBT) during your follicular phase? You're not alone. While elevated pre-ovulation temperatures can sometimes be explained by simple lifestyle factors, they can also point to something worth investigating. Here's what your chart could be telling you.

What Is BBT and How Is It Measured?

BBT is your body's lowest resting temperature, measured first thing upon waking, before any activity. For an accurate reading, you need at least four hours of continuous sleep, with the last hour uninterrupted. Any disturbance before measuring, such as getting up, talking, or even checking your phone, can affect the result.

Understanding BBT Across Your Cycle

Average basal body temperature during the follicular phase (before ovulation) typically ranges between 36.1 and 36.7°C. Shortly after ovulation, progesterone, a steroid hormone released from the corpus luteum, becomes the leading factor in the spike of BBT. This occurs in the luteal phase and drives BBT to increase an estimated 0.2 to 0.6°C throughout the luteal phase.

Studies found the average early-luteal phase temperature to increase 0.18°C from baseline in the fertile window. Early-luteal phase temperatures then continue to slightly increase until reaching a plateau of elevated BBT, before dropping back down to follicular phase levels as menstruation begins.

Interestingly, the thermal nadir (the lowest point of body temperature) was found to occur within 1 to 2 days after the LH spike in the follicular phase, approximately 1 day before the onset of ovulation.

Why Is My BBT High in the Follicular Phase?

High BBT during the follicular phase usually indicates an anomaly. Here are the five most common causes.

1. Lifestyle Factors

Everyday lifestyle factors can temporarily elevate your BBT readings. Known as disturbance factors, these can distort individual readings and lead to irregular spikes or drops on your chart. Typical causes include:

• Insufficient or disrupted sleep
• Waking up unusually early
• Shift work
• Unusual alcohol consumption
• Stress, emotional strain, or excitement
• Changes in environment (travel, holidays, climate changes)

These influences are perfectly normal and occur for nearly every user at some point. If your temperature is consistently high in the follicular phase, consider whether any of these factors were present the night before.

2. Early Pregnancy

If your BBT remained elevated from the previous cycle's luteal phase and hasn't dropped, this could be an early sign of pregnancy. During pregnancy, progesterone levels remain high, which causes BBT to stay elevated. On average, your BBT may rise by 0.2 to 0.3°C above your pre-pregnancy baseline. A sustained BBT elevation of more than 18 days after ovulation can be an early pregnancy indicator, though a test is necessary for confirmation.

3. Thyroid Issues

A consistently higher-than-normal average BBT might indicate a condition such as hyperthyroidism (overactive thyroid). Other symptoms may include:

• Longer menstrual cycles
• Light or scanty bleeding
• Increased nervousness or restlessness
• Rapid or excessive sweating

Important: While a higher BBT curve can be a clue, only a healthcare professional can confirm thyroid-related conditions.

4. Hormonal Imbalances

High, unstable, or erratic temperatures during the follicular phase can suggest irregular ovulation, high stress, or a "heat" condition often linked to infections or inflammation. If your chart looks consistently chaotic, hormonal testing may provide more clarity.

5. Endometriosis

A consistently high follicular phase temperature has sometimes been associated with endometriosis. While BBT alone cannot diagnose this condition, it can be a useful data point to share with your doctor if you're experiencing other symptoms.

What to Do Next

If you're seeing consistently high BBT readings in your follicular phase, start by ruling out external factors. Track your sleep, stress levels, and measurement timing. If the pattern persists across multiple cycles, consult a healthcare professional for further investigation.

Track Your BBT with Confidence

If you're ready to take a more accurate, effortless approach to BBT tracking, explore our range of fertility monitors designed to do the hard work for you:

• Daysy Fertility Tracker — a smart, app-connected device that learns your unique cycle and predicts your fertile days with 99.4% accuracy.
• Lady-Comp — a standalone fertility computer with the same accuracy, no app or subscription required.