The Better Mother by Jennifer van der Kleut

A modern spin on Fatal Attraction meets The Hand that Rocks the Cradle, perfect for fans of The Last Mrs. Parrish.

Still recovering from a devastating breakup, 34-year-old Savannah Mitchell has finally managed to put her life back together when she gets the shock of her life—after a brief fling with a man named Max, she is pregnant.
When she gets in touch to tell him, he reveals that he’s just gotten back together with his ex, Madison, and he will need time to break it to her. Surprisingly, Madison isn’t upset—in fact, she’s excited, and wants to help.
Max insists Madison has the best of intentions, but Savannah finds her efforts—popping by uninvited, demanding lifestyle changes, and pretty much trying to take over the pregnancy—anything but helpful. When Savannah finally stands up for herself, Madison’s treatment of her goes from casually cruel to downright dangerous.
All Savannah wanted to do was form a friendly co-parenting relationship with the father of her child—but his new girlfriend obviously has much more sinister plans in mind.

She has no plans to co-parent at all.