Baby showers, like many occasions in which gifts are given, can bring up a variety of etiquette questions on how to handle the details of giving---or even accepting---gifts. Addressing a baby shower card should be a simple and straightforward, although the proper way to address the card depends on the exact situation. Consider whether the shower is a girl's only or couples celebration, which parent you know, if the couple is married or living together or if the mother-to-be is doing it all on her own when addressing the card.
Address the outer envelope with names only if you're attending the shower, or with the full name of each parent and address if you're mailing the card. For a ladies-only shower, write the mom's first name if the shower is casual. Write her proper name if the shower is formal or if you only know her in a professional setting and feel more comfortable that way. Write both parents' names if the shower is a couples event. Alternatively, you could simply write "for baby" or write the baby's name if you know it.
Write "Dear (Mom's First Name)" inside the card if you're close to the expectant mother. Write both parents' names if it's a couples shower or if you're closer to the father. The name you write inside the card should be the same as the outer envelope, unless you only know the father and it's a ladies shower.
Send the card or put it with the other gifts at the shower.
Use your best handwriting. Include sincere well wishes for the parents and baby inside the card.
Avoid mentioning the father if he is not involved or if there are major problems between the parents. Baby showers are a time to celebrate the baby, not bring up harsh feelings or situations.