As a personal trainer, a woman and pro athlete I'm always for exercise during pregnancy. Strength training is a must and definitely beneficial for both mom and the baby, that being said, depending on trimester and individual physiology of each woman, there are certain exercises that should be either avoided altogether or modified. If the athlete is experienced (let's take a high level crossfit female athlete) she is unlikely to hit her stomach with the bar by performing a clean or snatch for example. Woman is very aware of her changing physiology and people who say there is a high risk of injury, simply don't understand that. Now, if we talk about absolute newbie or average gym goer, of course, there will be different limitations. It all depends on the person. Post pregnancy exercise is a very different topic that should be addressed more often. As once you are pregnant, your body changes forever and certain adjustments should be made to the training.