Totally agree with Squonk and Janitor! To their comments I would add:
Pretend that you find the foetus as fascinating as she does (in reality I think most expectant fathers don't properly get engrossed until after the birth). Be prepared for serious discussions over patterns of kicking, times of day, types of queasiness, position of kicks, shape of belly, position of stretchmarks... etc etc etc.
It is impossible for the father-to-be to do too much housework/cooking.
Tell her that you love her/still find her sexy/think she has never looked more beautiful as many times a day as possible. If you think you have said it enough times, say it once more. This goes double after the birth.
Be prepared to take advantage of the times she is feeling sexy, and be good humoured about the times when she doesn't. Most pregnant women get ragingly horny at certain points in the pregnancy and go completely off it at other points. Make hay while the sun shines!
If she says she is feeling crap/rough/knackered, do not respond by saying "oh yes, me too." Again, this goes double during/after the birth.
Make the most of being a couple while you still can. It is the last time it will just be the two of you.
Remember she loves you too - even if she forgets to tell you that sometimes. It is easy to feel so overwhelmed by pregnancy and motherhood that you forget you are still a wife/lover. This too will pass.