Should you ride a pregnant mare?

The answer is: Yes to a point. Mares (just like women) benefit from regular exercise (and in fact, in foal mares have raced and competed) up until the 6th to 8th month of pregnancy. Extreme exercise is not recommended for the first two months, but during the middle part of the pregnancy it's fine to ride a mare as normal. After that, exercise should be reduced and the mare should not be worked within three months of foaling because the foal can affect the mare's balance - and because really, why make her carry a foal and a rider. Saddle fit can also be affected.

And, of course, some mares show no sign of being pregnant until you wake up to find you have one more horse, and they often survive being worked as normal ;).