Burnished Flames by Amy J. White

Bethany thought her life had been perfect; dutiful wife to a hedge fund manager. Society lady – hostess, and organizer of charity events. Right up until her husband of almost a decade had brought divorce papers home so that he could marry his pregnant, nineteen year old mistress. Now Bethany has to make a whole new life for herself, which might be easier if her one night stand with former flame, Mark Stone hadn’t also knocked her up.
Mark Stone has only ever loved one woman, and when Bethany steps back into his life, bigger and better than before, he knows that he has to make her his. When she winds up pregnant, he sees it as a sign. And when it becomes clear that someone is trying to kill her, this military man will stop at absolutely nothing to keep her safe.