Currently, several players with Animal Husbandry skill over 50 (even over 70) are facing the similar problem. It was noticed that animals (horses and buffalo only) die without any reasons (healthy and fat). Miscarriages happen very often and sometimes lead to death of the mother, again without any visible reasons, all animals are with good reserves of fat, they are not venerable and groomed well. In other words, we see the high animal mortality not only with the pregnant animals. It's not related with the server reboots.
Wurm Unlimited: Dedicated Server - v. 1.3.5.5
Vanilla - No mods