Under Name Fields in the Styles and Standards section of WikiTree Help (http://www.wikitree.com/wiki/Name_Fields#Special_rules_for_required_fields):
"If an infant died before he or she could be named, use Unnamed Infant. Do not use Infant Boy, Baby Girl, etc. There is a gender field for this."
Additionally, you can enter a statement with more information in the Biography - i.e. "This male child was stillborn."