Hi there-
I worked out w/ Cathe videos throughout my whole pregnancy (modifying when needed and appropriate) and found it was pretty tough to increase muscle tone. But, I went into my pregnancy at pretty much my peak of fitness, eating really clean, high protein, and a lower body weight. Once I was pregnant, I really struggled w/ morning sickness for the first 2 trimesters and couldn't eat as clean (read: higher starchier carbs, less protein) of foods.
I was able to MAINTAIN a decent bit of muscle tone. But, what was a nice surprise was the tone I got in my legs from carrying all of that extra weight on top around. In my last trimester, I could see some quad definition that I didn't have before.
Also, another little bonus for muscle definition is post partum. Believe it or not, my upper body esp my shoulders and biceps saw some nice gains just from carrying the baby around. I guess it makes up for what happens to the abs after a baby. (But those do come back too, esp, if you are doing Cathe's ab workouts!!)
Good luck, Lynn M.