Was having a hell of a time judging how low of health one was when impregnating and also accidently killing it. What I found worked was to ride a direwolf and while it's attacking my host to be, shoot with whatever until the direwolf's cross turns orange. From there, I just chuckled spears at it until it turned red and then popped the pheromone. Worked 3/3 times in a row and solved my problem.