Basal body temperature (BBT or BTP) is the lowest
body temperature attained during rest (usually during sleep). It is usually estimated by a temperature measurement immediately after awakening and before any physical activity has been undertaken. This will lead to a somewhat higher value than the true BBT.
In women,
ovulation causes a sustained increase of at least in BBT. Monitoring BBTs is one way of estimating the day of ovulation. The tendency of a woman to have lower temperatures before ovulation, and higher temperatures afterwards, is known as a
biphasic temperature pattern. Charting this pattern may be used as a component of
fertility awareness. The BBT of men is comparable to the BBT of women in their
follicular phase
The follicular phase, also known as the preovulatory phase or proliferative phase, is the phase of the estrous cycle (or, in primates for example, the menstrual cycle) during which follicles in the ovary mature from primary follicle to a fully ...
.
Hormonal causes of biphasic patterns
The higher levels of
estrogen present during the pre-ovulatory (follicular) phase of the
menstrual cycle
The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls the production and release of eggs a ...
lower BBTs. The higher levels of
progesterone
Progesterone (P4) is an endogenous steroid and progestogen sex hormone involved in the menstrual cycle, pregnancy, and embryogenesis of humans and other species. It belongs to a group of steroid hormones called the progestogens and is the m ...
released by the
corpus luteum
The corpus luteum (Latin for "yellow body"; plural corpora lutea) is a temporary endocrine structure in female ovaries involved in the production of relatively high levels of progesterone, and moderate levels of estradiol, and inhibin A. It is t ...
after ovulation raise BBTs.
After ovulation, the temperature will be raised by at least , for at least 72 hours, compared to the previous six days.
The rise in temperatures can most commonly be seen the day after ovulation, but this varies and BBTs can only be used to estimate ovulation within a three-day range.
If pregnancy does not occur, the disintegration of the corpus luteum causes a drop in BBTs that roughly coincides with the onset of the next menstruation. If pregnancy does occur, the corpus luteum continues to function (and maintain high BBTs) for the first trimester of the pregnancy. After the first trimester, the woman's body temperature drops to her pre-ovulatory normal as the placenta takes over functions previously performed by the corpus luteum.
Very rarely, the corpus luteum may form a
cyst. A corpus luteum cyst will cause BBTs to stay elevated and prevent
menstruation
Menstruation (also known as a period, among other colloquial terms) is the regular discharge of blood and mucosal tissue from the inner lining of the uterus through the vagina. The menstrual cycle is characterized by the rise and fall of hor ...
from occurring until it resolves, which could take weeks or months.
As a birth control method
While avoiding pregnancy
Charting of basal body temperatures is used in some methods of fertility awareness, such as the
sympto-thermal method
Fertility awareness (FA) refers to a set of practices used to determine the fertile and infertile phases of a woman's menstrual cycle. Fertility awareness methods may be used to avoid pregnancy, to achieve pregnancy, or as a way to monitor gyneco ...
, and may be used to determine the onset of post-ovulatory infertility.
When BBT alone is used to avoid a pregnancy, it is sometimes called the Temperature Rhythm method.
Basal body temperature alone is most effective at preventing pregnancy if the couple abstains from intercourse from the beginning of menstruation through the third day after the basal body temperature has risen.
BBTs only show when ovulation has occurred; they do not predict ovulation. Sperm typically lasts for at least three days and can survive as long as a week,
making prediction of ovulation several days in advance necessary for avoiding pregnancy.
Effectiveness
There is limited evidence about the effectiveness of
fertility awareness family planning methods, some of which use basal body temperature as one component.
About 24% of women who use any type of fertility awareness program become pregnant during the first year, compared to about 85% of sexually active women who are not trying to prevent a pregnancy.
The
World Health Organization ranked fertility awareness methods, taken as a whole, as an "effective" method of preventing pregnancies.
The WHO placed fertility awareness methods in the third tier of effectiveness, after "most effective" methods such as
IUDs and "very effective" methods such as
combined oral contraceptive pills.
Trying to conceive
Couples that are trying to conceive can use BBT to determine when the opportunity for a pregnancy during this cycle has passed.
As a diagnostic test
For infertility
Infertility due to lack of ovulation is common. BBT charts can be used to identify when and whether ovulation is taking place.
Regular menstrual cycles are often taken as evidence that a woman is ovulating normally, and irregular cycles is evidence she is not. However, many women with irregular cycles do ovulate normally, and some with regular cycles are actually
anovulatory or have a
luteal phase defect. Records of basal
body temperature can be used to accurately determine if a woman is ovulating,
and if the length of the post-ovulatory (luteal) phase of her menstrual cycle is long enough to sustain a pregnancy.
For pregnancy
Most
pregnancy tests are not accurate until two weeks after ovulation. Knowing an estimated date of ovulation can prevent a woman from getting false negative results due to testing too early. Also, 18 consecutive days of elevated temperatures means a woman is almost certainly pregnant.
For estimating the timing of childbirth
Calculating the
expected due date for a pregnancy based upon the self-reported
last menstrual period is less accurate than calculating it based upon either BBT or ultrasound.
References
External links
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{{DEFAULTSORT:Basal Body Temperature
Fertility awareness
Gynaecology
Thermoregulation