The Memory Keeper's Daughter by Kim Edwards

Families have secrets they hide even from themselves... It should have been an ordinary birth, the start of an ordinary happy family. But the night Dr David Henry delivers his wife's twins is a night that will haunt five lives for ever. For though David's son is a healthy boy, his daughter has Down's syndrome. And, in a shocking act of betrayal whose consequences only time will reveal, he tells his wife their daughter died while secretly entrusting her care to a nurse. As grief quietly tears apart David's family, so a little girl must make her own way in the world as best she can.