Hope this helps:
Reproduction—The donkey is more prepotent [high in its ability to transmit certain characteristics to its offspring] but less fertile than the horse. It has 50% to 60% conception rate, compared to the horse’s average of 60% to 65%. The conception rate for mares carrying mule foals is about same as for horse foals, but for jennets carrying hinny foals the rate drops to about 25%.
Compared to a gestation period of 11 months for the horse, the donkey’s gestation period averages 12 months, but may vary between 11 and 14 months. The gestation period for a hybrid foal is usually intermediate between the parent species. Production of twins, although rare, is more frequent among donkeys than among horses.
The mule is a sterile hybrid, yet occasionally a mare mule will be fertile. The difference between the numbers of chromosomes in the cells of the donkey (62 chromosomes; 31 pairs) and the horse (64 chromosomes; 32 pairs) results in a mule or hinny with 63 chromosomes. This odd number is responsible for mule’s sterility—the donkey and horse chromosomes are unable to form matched pairs during the early stages of conception, resulting in the death of the reproductive cells.
http://www.ruralheritage.com/mule_paddock/mule_compare.htm