For those that have already had a child, you can probably ignore this post. For those that are planning to be fathers in the future, I thought I'd pass on a bit of advice that I've learned during the first trimester.
First, no matter how much you support your wife, you will not be able to offer her any words or actions that will make her happy that her belly is quickly growing. Even if you offer support or a simple factual statement such as "You're not getting fat, you're going to have a baby" after she complains that she feels fat. No words in the English vocabulary will be able to console your wife into thinking anything other than she's "getting fat", even when you are being quite honest and have nothing but the best intentions with your comments. I'm sure she appreciates your support, but when she's pregnant, she's not really in the mood to hear about it.
Second, the moment when your wife gets to the point when she no longer fits into her regular clothes, run (do not walk) to the nearest maternity store with a few hundred dollars in your pocket fully expecting to spend it all. While the cost of the clothes may smart for a day, the instant and future dividends when your wife feels better about herself and is able to actually breathe in her new clothes will last much longer. Trust me, this is one you want to store away. It will save you a lot of problems.