In foal
Definition 1 of 1
Pro Tip 1/3Species Specific
Primarily used for horses, donkeys, and zebras, not for other animals. 
Have you seen the zebra in foal at the zoo?
Pro Tip 2/3Not for Humans
Never used for humans; it's specific to equine pregnancy. 
It's incorrect to say, 'She's in foal' when referring to a pregnant woman.
Pro Tip 3/3Breeding Context
Commonly used by breeders and in veterinary context, indicating familiarity with equine terminology. 
The breeder confirmed that three of our mares are in foal.