Basal body temperature (BBT) and ovulation

What does your body temperature have to do with ovulation?

Basal body temperature (BBT) is the temperature of your body in full rest, and this temperature is influenced by the hormones that regulate your menstrual cycle.

During the first half of your cycle, the dominating hormone is estrogen which helps your ovaries prepare a mature egg for ovulation. After ovulation, the production of estrogen drops, and progesterone takes over to help prepare your uterus for the implantation of a fertilized egg.

Producing progesterone has the effect of increasing your body temperature – not so much that you can usually feel it, but enough to be measurable with a basal body thermometer. Hence the interest of measuring and charting your temperature: An increase in basal body temperature indicates that you have ovulated.

What does BBT charting tell you?

Because the temperature shift doesn’t show up on your chart until after ovulation, when you are no longer fertile, it cannot help you with intercourse timing. It will help you, however, with the following:

  • Getting familiar with your normal cycle. Not every woman ovulates on the 14th day of her cycle. After a few cycles of BBT charting, you will learn to know when to expect ovulation.
  • Alerting you to anovulation (failure to ovulate). If your charts rarely or never show the temperature shift indicating ovulation, you may not be ovulating at all. This will make it impossible to conceive and you need to consult your physician.
  • Identifying a luteal phase defect. The luteal phase (the time between ovulation and your period) needs to be at least 10 days in order for a fertilized egg to have time to implant in the uterus. Charting can help you see if your luteal phase is too short and you need to seek medical advice.

How to measure your basal body temperature

  • Use a digital thermometer with two decimals for maximum precision.
  • Measure your temperature as soon as you wake up every morning, before getting out of bed. Too much movement will influence your temperature and give inaccurate results.
  • Taking your temperature in your vagina or anus will give the most reliable results, but you may also measure in your mouth. If you choose the latter, tuck the thermometer well in under your tongue.
  • Take your temperature at the same time every day (+/- 30 minutes), including weekends.
  • Record your daily temperature on a chart. You can use a simple sheet of graph paper, a calendar, an Excel sheet, an online charting service such as FertilityFriend.com, or a smartphone app.

How to interpret your temperature chart

  • Your chart begins on the first day of your menstrual period (cycle day 1). This is the first day you have a fresh red blood flow (not just spotting).
  • Ovulation usually occurs the day before your temperature rises. The temperature rise after ovulation is usually about 0.4°F/0.2°C.
  • After ovulation, your temperature stays elevated. You must record higher temperatures for at least three consecutive days to be confident that you have ovulated.
  • If you are pregnant, your temperature will stay elevated beyond your usual luteal phase length.
  • If you are not pregnant, your temperature will usually drop when you get your period, or a day or two before.

Other tips on basal body temperature charting

  • Your temperatures will fluctuate from day to day – this is normal. Single temperatures don’t mean anything; you are looking for the biphasic (two-phased) pattern, with lower temperatures before ovulation and higher temperatures after ovulation.
  • Your temperature may be influenced by lack of sleep, alcohol, illness and many other factors. Note such factors on your chart, and don’t worry too much about single atypical temperatures.

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