Tracking basal body temperature, step by step

What is basal body temperature?

Your temperature follows a small daily rhythm and is lowest when you have rested. In the first half of a cycle, before ovulation, readings sit in a lower range. After ovulation, the hormone progesterone warms the body slightly, and the higher range lasts until your next period. When you chart this each morning, a shift appears.

The change is tiny, a few tenths of a degree, so ordinary thermometers are not precise enough, and a feverish or restless night can mask it.

What do you need?

• A digital thermometer that reads to two decimal places (0.01 °C or 0.01 °F), or a fertility thermometer made for the purpose.
• A chart on paper or in an app. Any simple table of date and temperature works.
• A few minutes each morning, and some patience, because the pattern appears over several cycles.

Some wearable devices and smart rings estimate temperature overnight. They can be convenient, and they vary in accuracy, so check how a product explains its method.

What is the daily routine?

1. Take your temperature at the same time every morning. Do it before you sit up, drink, eat or talk.
2. Use the same thermometer and the same way of taking it. Under the tongue, vaginally or rectally all work if you stay consistent.
3. Write the number down right away. Add notes for anything that can change it, such as a late night, illness, alcohol or a disturbed sleep.
4. Start on day 1 of your period and chart every day of the cycle.
5. Mark the first day of your next period and begin a new chart.

How do you read the chart?

Before ovulation, temperatures usually stay in a lower range. After ovulation, progesterone raises your temperature by a small amount, often about 0.2 to 0.5 °C (0.4 to 1.0 °F), and it stays higher until your next period. If your temperature stays high for more than about two weeks, that can be an early sign of pregnancy.

To spot the shift, look for a run of higher readings that stays above the earlier ones. A single high reading means little, because sleep, illness or alcohol can raise it. A common rule of thumb is to look for three days in a row that are higher than the six days before. Draw a line just above the highest of those earlier six readings, called a coverline, and see whether later readings stay above it.

Here is a simple picture of what a pattern looks like:

These numbers are only an example. Your own range matters more than the figures.

What can BBT do, and what can it not?

The rise happens after the egg is released, so a chart tells you that ovulation probably happened and roughly when. Several months of charts can show whether you tend to ovulate earlier or later in your cycle, and that helps you time sex in the following months.

It is a poor tool for predicting the fertile window in the same cycle, because by the time the temperature rises, the most fertile days are behind you. Combine it with ovulation tests and cervical mucus if you want to look ahead. The signs of ovulation guide compares the methods.

A chart also cannot prove that an egg was released, that the tubes are open or that sperm are healthy. It is one clue about one part of the process.

How do you use your charts over several months?

Look back at your last three or four charts and note the day of the rise in each. If the rise tends to come around the same cycle day, you can plan the next window by counting five days back from that day. If it jumps around, widen your window and add an ovulation test. The ovulation day guide explains the counting, and the best time to conceive calculator lets you enter your own cycle length and luteal phase.

What makes a chart messy?

Many ordinary things blur a chart:

• Alcohol the night before, a late dinner or a disturbed night.
• Illness or fever, and some medicines.
• Shift work, travel and time zone changes.
• Waking at different times each day.
• A warm room, heavy blankets or a partner who shares a bed with a lot of heat.

Aim for at least three to four hours of unbroken sleep before you measure. Take your temperature at the same time even on weekends, or note when you did not. Leave a gap or mark the day if you were ill or slept badly, instead of guessing.

Is BBT right for you?

BBT suits people who like data and routines, and who are willing to look at a chart for a few months. It can feel like a lot if you already find tracking stressful. Stop charting if it makes you anxious, because other ways of timing sex work well. Intercourse every two to three days across the cycle covers the fertile window with no tracking at all.