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Top things to know about basal body temperature:
Basal body temperature (BBT) changes once ovulation occurs
This temperature change is caused by hormonal changes
BBT is not very helpful when trying to conceive because BBT increases after ovulation has already happened
Basal body temperature (BBT) is the body’s temperature at rest (1). Changes in BBT can identify when ovulation has happened (1,2). After an ovary releases an oocyte (immature egg), there is a rise in the hormone progesterone (1). This sharp increase in progesterone causes a slight rise in your body temperature, from 0.4 to 1.0℉ (1,2). Some people notice a slight drop in temp 12 to 24 hours before ovulation (1).
To monitor BBT, it is recommended to take your temperature immediately after waking up while still laying in bed and before sitting upright (1).
BBT changes are very small (1). Using a special basal thermometer that measures tenths of a degree can help catch those changes (1). Oral, rectal, and vaginal temperatures are all suitable for BBT charting, but using the same site every day is important (1).
BBT is not very helpful in predicting ovulation and timing sex before ovulation (2). An oocyte must connect with a sperm within 24 hours of ovulation to survive (1,3). BBT increases after ovulation has already happened (1,2). This may not provide enough time to plan for sex.
BBT is not considered a reliable tool when trying to conceive, and is not recommended by The American Society of Reproductive Medicine (4). It requires strict, daily charting, and BBT changes can be missed because of common life events such as interrupted sleep, being ill, having a fever, and even stress and alcohol (1,2,4).
That said, some people may find it helpful to measure BBT. It can be an affordable option for monitoring when ovulation has happened. If BBT is taken and charted daily as directed by the manufacturer, it has the potential to pinpoint cycles where an ovary does not release an egg (4).
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