I bought my horse from a woman who bought him as an unborn foal! He didn’t work out for what she wanted (a dressage horse) so he’s a jumper now. I don’t know if I would personally do it but I don’t see an issue
Hope this helps. Or you can wait untill the foal is born and you can advertise that this mare with her qualities and qualifications and this stallion with his qualities and his qualifications is expecting a baby.