One time I was pregnant. There were some simple things that I really wanted from my partner but at the time, I was too emotional to ask for them in a constructive way. Being a parent is a radical experience, and the starting point is the pregnant part. If you can provide an environment that is comfortable to your pregnant partner, the pregnancy, the birth, and the first months of insanity can be greatly improved. I believe that a pregnant woman's needs are totally within the scope of what you can do.
Step 1: Take notice of the amazing power of her body
Most everyone wants their work to be appreciated, especially in relationships. When someone is working hard, it makes the experience more enjoyable to know that it is not in vain. Your lady is creating a human being without even knowing what she is doing. The baby is being made from parts of her body. It's totally insane and powerful, and if just once a day, you let her know that her body is amazing and doing a good job, you will make her happier.
Step 2: Water and vitamins
Pregnant women are thirsty. They need beverages wherever they go. It would be really really sweet if you could make sure that she has water. She might already be on top of this water-having need, but I guarantee you that she will forget her water bottle at least one time. If you are the one that produces a little bottle of water for her from your bag or car or whatever, you will earn yourself some hugs and you will avoid the panic and crazed behavior that happens sometimes when a pregnant woman gets thirsty and can't find water. I think it's totally an animal thing, but never go walking with a pregnant woman unless one of you is carrying water.
Vitamins. Likely, she will take some kind of supplemental vitamin while pregnant. If you can make sure that she takes her vitamins every day with maybe even just an email or text message, she will feel like she is part of a well-oiled pregnancy team.