There's a halfway point where you only get to pick one trait, even though the game will tell you the pregnancy went smoothly. A truly difficult pregnancy won't let you pick any traits at all. And, of course, a great pregnancy lets you pick both traits. Also, in my experience, the only thing that really matters during the pregnancy is that the mother gets enough rest. If her energy bar goes into the yellow, she's going to have some trouble. Depending on how many times and how bad it gets, that's when the pregnancy will be a difficult one.