Most people write the baby message to the parents, congratulating them on being parents. You can write a brief message to the baby, and then address the parents. You can either write a fun message or a sincere one, writing down your feelings about parenthood. Maybe the parents will keep the baby shower cards until the baby gets older, and that will be even more fun.Visit http://www.bukisa.com/articles/290737_what-to-write-in-a-baby-shower-card-message-examples for both serious and fun ideas on how to write a baby shower card.
There are many ways to express joy through a baby shower card. Explain the the bundle of joy is going to be loved and that people are looking forward to the birth. Also that you wish them a lifetime of happiness.
According to Best Card Messages, it's customary to congratulate the expectant couple and to wish health and happiness to the whole family. You may also consider offering any expertise you have to offer. See sample messages at http://www.bestcardmessages.com/baby-card-messages.html.