Here's why:
* Natural instincts: Horses, like most mammals, have strong instinctive avoidance of incestuous behavior.
* Lack of sexual maturity: A colt foal is not sexually mature until around 18 months to two years old. At that age, they will likely be separated from their mother anyway.
* Social hierarchy: A mare's dominance over her colt will also discourage any attempts at mating.
* Physical cues: A pregnant mare's physical changes, such as swollen teats and a rounded belly, would likely not be attractive to a colt.
Therefore, it's extremely unlikely that a horse colt foal would attempt to mate with his mother, even if she is pregnant.