The hormone progesterone is only secreted in the second half of the cycle after ovulation. It has many effects on the body, one of which is to prepare the lining of the uterus (endometrium) ready for a fertilised egg to implant, and another of which is to raise the basal body temperature ever so slightly. All you are doing when you take your temperature is estimating whether your progesterone is likely to be raised or not. It is notoriously difficult to produce a ‘perfect’ temperature chart showing the classical dip just before and then the rise after ovulation. Many things have an effect on basal body temperature, as measured by an oral thermometer. The very first thing you do each morning needs to be to put a thermometer in your mouth – even before you get out of bed.