When one speaks about basal body temperature (BBT), one is actually talking about one’s natural body temperature at rest. The basal temperature changes based on a number of factors, including:
An implantation dip is a one-day drop in temperature on a basal body temperature chart that occurs about one week after ovulation. Usually, a dropping temperature is a sign that one’s period is coming or has already started. One’s period should not come just seven to 10 days after ovulation, so one wouldn’t expect a temperature drop at this time.
With an implantation dip, the temperature will rise right back up the next day. The fall lasts just one day.
The dip may just be slightly lower than the rest of one’s post-ovulation temperatures. Or, it may drop below the coverline on one’s fertility chart. The coverline is an imaginary horizontal line separating where one’s temperatures on average were before ovulation as opposed to where, on average, they are after ovulation.
The dip appears during the luteal phase, the time between ovulation and one’s expected period. Also, implantation of the embryo usually occurs between days seven and 11 of the luteal phase. This is the precise reason why some people attribute this sudden one-day dip in temperature to implantation.
Speaking in terms of numbers, a woman’s normal non-ovulating temperature is between 96 and 99 degrees Fahrenheit, depending on the individual. Following the release of the egg, BBT increases by about half a degree in almost all women. The hormone progesterone, secreted by the ovary after ovulation, heats things up; it also prepares the uterine lining for a possible pregnancy. The body temperature will remain roughly half a degree higher until right before menstruation, when it will return to normal.
Because the spike in body temperature at ovulation is so small, one needs a special basal thermometer (available in drugstores) to measure it. A basal thermometer records temperatures in increments measuring one-tenth of a degree as compared to the two-tenth increments on normal fever thermometers.
Going deeper into the number games concerned with rising and falling BBT,
If the fertilized egg doesn’t implant, the progesterone level and basal body temperature will decrease before menstruation.
Hence, the dynamics of the basal body temperature might be an indication for ovulation and indicator of the woman’s fertile window.