Her by Harriet Lane

Emma, pregnant with her second child, is feeling isolated and overwhelmed with her daily life as a stay-at-home mom to 2 1/2 year old, Christopher. She's an easy mark for Nina, who knows Emma from when they were younger, although Emma doesn't remember her. Nina has great disdain for Emma and blames her for something that happened in the past. Slowly, Nina starts exacting revenge in the littlest ways, but the reader fears for Emma and her family, who have no idea that Nina wants to destroy them.