There is a mid-cycle shift in the basal body temperature (BBT) after ovulation in women. This is caused by:

Basal Body Temperature Biphasic changes in BBT are characteristic of the ovulatory cycle Mediated by alterations in progesterone levels. Secretion of progesterone following ovulation causes a temperature rise of about 0.5 to 1degF over the baseline temperature of 97degF to 98.8degF that is typically recorded during the follicular phase of the menstrual cycle. Ovulation is assumed after 3 consecutive days of raised temperatures.