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Charting Fertility Using Basal Body Temperature

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The female body is complex, and getting pregnant can be a challenge for some couples. Other couples would like to start a family as soon as possible, while some women may simply want to understand more about their fertility and how their body works. The most effective, least invasive means of understanding a woman’s fertility patterns is by charting basal body temperature.

Basal body temperature is defined as the lowest temperature the body reaches during rest. Charting one’s temperature daily provides a wealth of information regarding a woman’s fertility due to the fact that changes in hormone levels that occur during the menstrual cycle affect basal body temperature. The first day of a woman’s period is considered the first day of the new cycle. The time from cycle day one until an egg is released at ovulation is known as the follicular phase of the menstrual cycle. During the follicular phase, estrogen levels are high, and one or more eggs may begin to develop in the ovaries. The high level of estrogen present during the follicular phase keeps the basal body temperature relatively low.

When an egg has developed sufficiently, changes in hormones allow for the egg to be released from the ovary. The release of the egg is known as ovulation. Following ovulation, the area of the ovary that released the egg produces large amounts of the hormone progesterone, while estrogen levels decline somewhat. Progesterone has a dramatic effect on basal body temperature, which results in a rise of 0.5° to 1.0°F (0.25° to 0.5° C). This effect of progesterone results in what is known as a biphasic temperature pattern. That is, during the follicular phase, basal body temperature is low; following ovulation, temperatures rise and remain elevated for the second phase of the cycle.

After ovulation has occurred, progesterone remains elevated for ten to fourteen days. This portion of the menstrual cycle is known as the luteal phase. If a pregnancy does not occur, then at the end of the luteal phase, the levels of progesterone being released from the ovary decline, resulting in a decline in basal body temperature. Once progesterone levels drop sufficiently, the lining of the uterus is shed, thus beginning the next cycle.

Charting basal body temperature is an easy and usually reliable way to determine if ovulation occurs. Although basal body temperature cannot predict when ovulation will happen, this information can help identify fertility problems such as anovulation (no egg released) or luteal phase defects.

For the purposes of charting fertility, the temperature is typically taken orally immediately after waking by using a basal body thermometer. Basal body thermometers are more accurate than typical fever thermometers, and this is important in being able to detect the small shift in temperature that occurs upon ovulation. Although changes in environment do not usually affect basal body temperature, alcohol and some medications may have an effect. Consistently charting one’s basal body temperature is a safe, easy, reliable way to gain considerable insight into one’s fertility.

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