Basal body temperature (BBT) is defined as the lowest natural, non-pathologic body temperature recorded after a period of rest. Women have used charting average basal body temperatures over the length of a menstrual period has been a tool to determine if ovulation has occurred or not. Basal body temperature measurements have been used for decades to help women optimize the timing of intercourse during a fertile window. This unique methodology was also prevalent before the invent of oral contraception pills, as it aided women in an attempt to avoid fertilization during a fertile period. Still to this day, women use this seemingly simple tool for planning purposes.
Ideally, charting the daily basal body temperature will show a noticeable increase of approximately 0.5 to 1 degrees F shortly after ovulation. This increase then sustains until the start of menstruation in which BBT then decreases back to baseline. Unfortunately, accurate results are dependent on consistent measurements every day, which can be challenging for many women. For most accurate results, temperatures need to be recorded at the same time every day, immediately upon waking up. Measurements are also subject to environmental influences such as a fever secondary to an infectious process, emotional stressors, alcohol consumption, and the addition or discontinuation of oral contraception from a daily regimen.
Additionally, it is essential to understand the risks associated with using basal body temperature charting as a method of contraception. BBT measurements help aid women who are trying to conceive by tracking a cycle, but the mechanism does not protect an individual from sexually transmitted infections or unwanted pregnancy. Because of the many environmental factors that can alter BBT, it offers limited protection from pregnancy. Those who are uninterested in conceiving should stick to other hormonal contraception methods such as oral contraceptive pills, patches, injections, or implants. Condoms are also options for barrier protection.
Directly after ovulation, the corpus luteum is formed. This vital hormone-secreting organ that is comprised of follicular theca and follicular granulosa cells releases progesterone to prepare the body for the potential of pregnancy. One of the preparation mechanisms causes progesterone to act on the hypothalamus to set higher basal body temperatures. Resultingly, the luteal phase of the menstrual cycle is associated with increased feelings of warmth and perspiration. Progesterone and body temperature remain relatively high throughout the rest of the cycle, and will eventually decrease with menstruation if no pregnancy has occurred. A basal body temperature elevation that does not return to baseline with expected ovulation could be an early indication of pregnancy.
Follicular stimulating hormone (FSH) levels are typically elevated in prepubertal females as their immature bodies are developing. However, the onset of puberty increases levels of estrogen, leading to new luteinizing hormone (LH) surges just before ovulation. LH concentrations then become resultingly higher than FSH. This LH surge is the precipitating factor for an ovary to release one egg (sometimes more) each month. Due to the inherent relationship of basal body temperature variations around ovulation, understanding the basic development of menarche before research on basal body temperatures is important.
Interactions between three different organ systems are necessary for ovulation and the subsequent BBT fluctuations seen around the physiologic process. The hypothalamus-pituitary-ovarian axis is the foundation of menstrual cycles in women. The hypothalamus, a unique endocrine organ that secretes neurohormones, releases gonadotropin-releasing hormone (GnRH) in a pulsatile fashion. GnRH then acts on the anterior pituitary gland to release LH and FSH. The release of these hormones on the ovaries induces follicle maturation and ovulation, altering basal body temperatures.
The function of basal body temperature tracking relative to ovulation allows a woman to understand her menstrual cycle for various reasons better. If functioning as a birth control method, women should avoid intercourse from the start of menstruation until approximately three days after the spike of BBT. These three days are only an approximation as the BBT may not have a distinct elevation until three days after ovulation in some women. In women who have irregular menstruation, basal body temperatures are perceived not to be as accurate secondary to the unpredictability of ovulation timing.
Shortly after ovulation, progesterone, a steroid hormone released from the corpus luteum, becomes a leading factor in the spike of basal body temperature; this occurs in the luteal phase and drives BBT to increase an estimated 0.5 to 1 degree F throughout the luteal phase Average basal body temperature during the follicular phase (before ovulation) averages between 97.0 and 98.0 F. One study with a wrist wearable sensor found the average early-luteal phase temperature to increase 0.33 F from baseline in the fertile window. Early-luteal phase temperatures then continue to slightly increase until reaching a plateau of elevated basal body temperature. The elevation then drops back down to follicular phase levels as menstruation begins Contrarily, thermal nadir (the lowest point of body temperature), was found to occur within 1 and 2 days after the LH spike in the follicular phase, approximately 1 day preceding the onset of ovulation.
A woman can test her basal body temperature using a simple thermometer that is accurate to 1/10th a degree. If used, however, a woman must be sure to measure her BBT at the same time each morning. Recording the temperature before eating and drinking is also recommended as these factors can alter a resting temperate. Consistent morning measurements have been proven to be the most accurate. A commercially available vaginal temperature sensor product has a proclaimed 89% accuracy in the prediction of ovulation. Other temperature sensors utilize thousands of daily and nighttime measurements as they are easy to use sensors worn under the arm.
The interactions mentioned above regulate basal body temperature between organs and their resulting menstrual cycle hormones. Starting in the hypothalamus and ending with the release of progesterone, which promotes higher resting body temperatures, the ovulatory cycle significantly contributes to fluctuations in resting body temperatures.
Basal body temperature tracking has been a natural and straightforward tool women have used for decades to determine the cessation of ovulation. Since the discovery of the physiology behind BBT, women have been able to use the information for intercourse planning. Whether used as an aid in identifying a fertile period or rather to prevent conception, the natural tool has proven somewhat advantageous. However, various studies have expressed the viewpoint that basal body temperature measurement is an unreliable technique to determine accurate ovulation timing. It has been predicted in some studies to be only approximately 22% accurate in detecting ovulation. Although research outlets have determined the unreliability of basal body temperature measurement, the principle is still widely used and accepted by women to this day.
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