Found: His Royal Baby (Harlequin Romance)

Crown Prince Dane of Montenevada has just heard the rumors of a secret baby of regal birth. With the kingdom in an uproar, his only choice is to confront the sister of his sworn enemy—the woman who still haunts his dreams. Alexandra Acredonna has always feared her precious son would be taken from her. Now Dane is furious that she's kept the result of their lost week of passion from him. He offers her an ultimatum, one that will keep her identity hidden from their feuding families. But Alexandra wants more: a place at his side…as his beloved bride.