Place stallions and mares in separate stable groups, including any visiting mares you board. Players report pregnancies before they even built a breeding station. Once there is space in another group, look for the group control in the horse's Management menu.
Set up space before buying a stallion
Check whether your yard has a separate group with room before bringing a stallion home. If your current stable has only one group, make the needed space first or buy a mare instead.
Change the group from the horse's menu
In the Steam discussion about unexpected pregnancies, players point to the Management menu on the individual horse. Select the destination group, then check the horse's assignment before ending the day.
Check where you house boarders
Review which groups your visiting mares use. A player reported a boarded mare becoming pregnant and leaving early. Recheck assignments when a new boarder arrives or when you rearrange horses.
FAQ
Can a mare become pregnant without a breeding station?
Players report that it can happen when a stallion and mare share a group.
Where is the group control?
Player reports place it in the individual horse's Management menu.
Do visiting mares need separate space too?
Yes. Keep visiting mares out of a stallion's group if you want to avoid pregnancy.