A good starting point is at Which focal-length lens is usually used for portrait photography, and why?
The additional consideration with a maternity shoot is this: How much do you want the baby bump to bulge in the photos?
If you use a wider lens, such as a EF 35mm f/2 and shoot very close to your subject the same effect that enlarges noses relative to ears on faces will make the part of the belly closest to the camera appear to bulge more than it already does. Maybe you want that effect, maybe you don't. It is an artistic decision. If you compose with the belly closer than the subject's face, you can avoid the facial distortion while still geting the benefit for the belly if you want. For environmental shots this will be less of a concern as your subject will likely be further from the camera.
A longer lens, such as the EF 85mm f/1.8 will allow you to shoot from a little further away which will serve to compress the bulge of the baby bump.
In between these two is any of the 50mm lenses. I own both the EF 50mm f/1.8 II and the EF 50mm f/1.4 versions. If you are planning on doing any manual focusing or fine tuning, you want the f/1.4. Additionally, unless it is used wide open at f/1.8 where it is a little soft overall, bokeh is a little harsh with the f/1.8 II due to having only five aperture blades.