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Pregnancy is most likely to be achieved by having intercourse just before the time of ovulation, or the point in a woman’s cycle when the egg is released. Knowing exactly when ovulation occurs, so intercourse can be optimally timed, will improve a woman's chances of getting pregnant. Basal body temperature (BBT) is the base body temperature of a woman throughout her menstrual cycle. A rise in BBT is a good indicator of ovulation, because this temperature rise is directly associated with the onset of ovulation, meaning BBT and ovulation prediction go hand in hand.
It is necessary to understand the female reproductive cycle to utilize BBT and ovulation prediction. A woman’s reproductive cycle usually runs from 28 to 32 days. It begins on the first day of menstruation, the five- to seven-day period when the lining of the uterus is released, and ends on the first day of the next menstruation. The events of the cycle are primarily controlled by the hormones estrogen and progesterone. Estrogen works to nourish and enhance the lining of the uterus during the first part of the cycle and keeps BBT low relative to the end of the cycle.
During the second part of the cycle, progesterone is released. This hormone prepares the uterus to receive the egg and also causes basal body temperature to rise in preparation for the egg. This temperature rise coincides with ovulation, making it the key to using BBT and ovulation prediction to improve the chances of pregnancy.
The best time to try to get pregnant is two or three days before ovulation. A rise in basal body temperature predicts when ovulation has occurred, so the most fertile days have already passed when the BBT rise is observed. It is therefore necessary to chart, or record daily, BBT for a few cycles to determine the most consistent and likely day for ovulation, and from there predict the most likely fertile days for the next cycle.
To observe the change in BBT and calculate ovulation, it is best to record the temperatures on a chart, or ovulation calendar, for each day of the entire cycle. Several websites provide printable BBT charts or provide online charting. A BBT thermometer is necessary because it can measure temperature changes to 0.1 degree. To increase accuracy, the BBT reading should be taken at the same time every day. The morning, before getting out of bed, is the best time to record the temperature because it is not artificially elevated by movement; the reading can be done orally, rectally or vaginally but, for consistency, it should always be done the same way.
The first temperature reading for an ovulation calendar should be taken on the first day of menstruation, considered Day 1 of a cycle. The temperature should then be taken every morning until the first day of the next menstruation. Typically somewhere between Day 10 and Day 14, a small decrease in temperature may be observed, followed by a temperature spike of 0.2 to 0.6 degree. The day of this spike is the day of ovulation. The decrease in BBT should not be mistaken for ovulation and not all women observe a temperature decrease.
In some women, the basal body temperature remains constant and then spikes at ovulation, without being preceded by a BBT decrease. If the BBT increase was observed on Day 13, then the most fertile days for that cycle were on Days 10 or 11 through Day 13. Ovulation is fairly consistent, so the assumption should be that, in the following cycle, these same days will be the most fertile days. If the BBT remains elevated and the next menstruation does not start, then pregnancy may have been achieved.
BBT and ovulation prediction can be coupled with other tools, cervical mucus changes and ovulation predictor kits (OPKs), to further ensure that the most fertile days have been accurately predicted. Cervical mucus changes during the reproductive cycle and can be tracked on the ovulation calendar along with BBT. During the first part of the menstrual cycle, the amount of cervical mucus is small and the consistency of the mucus is thick. Close to ovulation, on the most fertile days, the mucus will increase in volume and will thin out and become stretchy.
Ovulation predictor kits are helpful tools to couple with BBT and cervical mucus charting. These kits test the urine for the presence of luteinizing hormone, which rises just before ovulation. Once a few ovulation calendars have been completed and the typical day for ovulation is determine, an OPK can be used to ensure that, indeed, the most fertile days have been accurately predicted.