if you get your AF the temperatures which are above your coverline will slowly go down. for example you've ovulated at day 14 you're temperatures will rise (0.3 degrees celsius or about 0.5 degrees fahrenheit) above the temperatures you've had before. Then they will stay high(ish) and then 3 days before your AF is due it will go down, lets say (this is in celsius) you've had 36.6, you will get (at the end) 36.6 36.4 36.2 and then AF starts. if you are pregnant you would get (for example) 36.6 36.5 36.7 36.6 etc etc so it will not really drop.
mind you this is only an average example and will depend on your personal cycles how they will turn out.
But with anyone, while being pregnant the temperature is always 0.3 degrees (celsius) higher than someone who is not pregnant or has not ovulated.