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What Is Basal Body Temperature?

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Your basal body temperature is the temperature of your body when it's completely at rest—that means when you're lying perfectly still in the morning, before you've had anything to eat or drink, before you've spoken or even moved.

a woman holding a basal body thermometer up

Understanding and charting your basal body temperature can play an important role in your overall fertility, as a raised basal body temperature can mean that you're ovulating. And knowing when you're ovulating means being able to make the best plans for conceiving.

How Does It Work?

To understand your basal body temperature, we need to take a look at progesterone. Progesterone is the hormone that helps prepare the lining of your womb in the event of a pregnancy, and therefore its levels slightly increase every time an egg is released from your ovaries. Every time your progesterone numbers increase, your basal body temperature increases. Though the change can be miniscule (as little as 0.4 degrees), it can be a sign that you're ovulating. That's why it's so imperative to get a good basal body thermometer, and know how to use it correctly.

Creating a Basal Body Temperature Chart

Though every woman is different, ovulation generally occurs in the middle of your cycle, and you are at the peak of your fertility in the couple of days before, and directly after, ovulation. This is the key time for sperm to fertilize your egg, and so being able to pinpoint the days you're ovulating, through charting your basal body temperature, can become an invaluable tool. On the first day of your next period, begin taking your basal body temperature at the same time each morning—before you get up, eat, or drink. Be sure to take it at the same time each day, since it generally rises by about 0.1 degrees every half hour. The numbers we're talking about here are quite small, so such a variation would make a big difference in the legitimacy of your chart. Start noting your result for a few months, and you will soon begin to see a pattern of temperature fluctuations, indicating your ovulation dates. If your basal body temperature is raised for three to five days, you’re ovulating. If it's raised for more than 18 days, congratulations, you are probably pregnant!

How Reliable Is A Basal Body Thermometer?

Basal body thermometers are readily available from your local pharmacy or online. For best results, keep it next to your bed, and pop it in your mouth as soon as you wake up. It sounds easy, but you must be regimented about it in order to get an accurate representation of your basal body temperature, and thus your ovulation schedule. It's important to keep in mind that many other things raise your body's temperature. Things such as illness, fever, alcohol or painkillers will influence the validity of your temperature. Also, you will only begin to see a pattern after a few months (if it all), so it's not something that will help you immediately. That means that if you have an irregular cycle, you might struggle to gain any benefit from a basal body temperature chart.

 
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