Being a trainer and having recently given birth myself, I firmly believe that leading a healthy and active lifestyle while being pregnant has very positive effects on the labor and delivery as well as recovery. As mentioned above, if your client is active before getting pregnant they should be able to maintain that level of activity without any negative effects on the pregnancy. The exceptions are of course if the client is experiencing any complications with their pregnancy and need to follow the contraindications related to that complication. The important thing to keep in mind is that the guidelines are just that and every client will be different. If your client was not active prior to being pregnant, it is important to introduce activity that will not overwhelm them. This would not be the time to begin metabolic circuit training. Some light resistance training, yoga, light cardio, etc would be appropriate.
On the day I gave birth I taught and participated in 2 boot camp classes, a zumba class and trained 2 clients before I realized I was in labor and had to go to the hospital. I had a very healthy little boy without any difficulties in the delivery. I recovered so quickly compared to friends who were not active during their pregnancies. I also did not gain a lot of weight and lost it all within 2 weeks.
I am so glad I listened to MY body and remained active and ate a healthy diet. Encourage your clients to listen to their bodies and remember there is no one size fits all answer.