Do you have a client who is pregnant and in her 40s?
There are two parts to the answer.
The first is that the American College of Obstetricians and Gynecologists have been putting out guidelines for exercise during pregnancy. These have changed over time as research has given better and better information, and I think these guidelines are going to be the best go to resource for general information for exercise and pregnancy.
However, all pregnancies are specific to the person who is pregnant, and what that person will choose to do or not do will depend on many factors: what they did before the pregnancy, their fitness level, their medical history, other conditions, and so on. And of course a pregnancy develops over the 9 months, and as the woman’s body shifts so will her exercise choices and intensities most likely shift. It is always helpful for the woman, her medical guide, and her trainer to work together to find what works best for her.