Until You're Mine by Samantha Hayes

Claudia Morgan-Brown is soon to have her first child when she hires nanny Zoe Harper. Claudia lives with her two young stepsons and husband, James, who is away weeks at a time with his job in the Navy. However, Zoe is not who she seems and is spying on the family. The reader also knows that she's planning leaving her job when the baby is born. Meanwhile, married detectives Lorraine and Adam are investigating a series of murders of pregnant women around Birmingham. As the story progresses, the reader wonders if Claudia will be next or if the police will be able to stop the brutal deaths.