If a woman has a temperature at the time of ovulation does it decrease her chance of getting pregnant.?

Maybe: Only if the illness caused a fluctuation enough to delay the ovulation. It is possible also that the increase in temperature night be hostile to the sperm, and that could also decrease the chances.

Unknown: Our experience alone with ivf shows a very low pregnancy rate in patients with fever during the follicular or egg development phase of the cycle. Probably natures way of protecting the species.