Sleep With Me by Joanna Briscoe

Richard and Lelia's child is conceived in a moment of giggling chaos as they dress for a Christmas party. They arrive rudely late and still glowing, and barely register a slight, drab woman in the hall. Sylvie. As their baby grows, so does the presence of Sylvie - she seems to be nowhere, yet everywhere, harmless yet sinister. Richard is seduced by her subtle, inexplicable charm, while Lelia, struggling with Richard's sudden ambivalence towards their baby, finds that she is haunted by painful memories. And Sylvie remains as invisible as she wants to be - that is the source of her power. Beware of mice...;