Secrets lead to conflict. Secrets can be so delicious and really add to a story. Everyone has secrets–whether it is desires, things they don’t want others to know about them. The secret should tie into the plot of the story. It often will be what the plot revolves around. Don’t develop a secret without using it. Again when coming up with a secret a writer goes back to the character’s background–what made him what he is today.
Example: IN A FAMILY FOR TORY, my March 2004 Love Inspired, the hero was driving the car when it wrecked killing his wife and injuring his daughter permanently. This wasn’t something he let people know. He didn’t tell the heroine who he had married until they had been married a while. That one secret drove his characterization and how he dealt with life.
So have fun and come up with something one of the characters is desperate to keep from the others. Secret baby story lines are popular because readers like the idea of a big secret being withheld from other people.