When the Bough Breaks — Lois Duncan

Kathryn and Robert Michener have been married twelve years when she learns of his affair with another woman. Her pride is shattered, and although Rob swears that Barbara means nothing to him, Kate starts divorce proceedings immediately…

Now, three years later, Kate and the children have settled in Albuquerque, New Mexico. She is achingly lonely, despite her job in an advertising agency, and often wished she had yielded to Rob's pleas for forgiveness. (But she must not think of Rob, who married Barbara soon after the divorce.)

It is out of loneliness that Kate accepts her first date with Dan McCown, an artist at the agency. They continue to see each other after office hours, and their chaste dinner dates inevitably lead to a deeper involvement -- until Kate discovers, to her dismay, that she is pregnant.

The decision she now faces is one few woman would envy. Although Dan offers to marry her, Kate knows he is far too young to be burdened with a ready-made family. And the very idea of abortion is repugnant to her.