The Fifth Child by Doris Lessing

The happy couple in 1960's middle class England has planned on 6 children from the day they got married. The first four come in a whirlwind and without financial and physical help from their parents, they would have gone under. However, their home becomes the gathering place of multiple families, bringing together a diverse group and creating a haven.

From the beginning of the pregnancy with their 5th child however, their world tilts a bit. After the birth, it wobbles along a bit until everyone can see that the child is not normal. He seems to be a throwback to an earlier more primitive species, but how can that be? And regardless of what he is, how do you manage a child who's very presence threatens to destroy the happy family life they've created?