Someone else's life

When seventeen-year-old Rosie's mother, Trudie, dies from Huntington's disease, Rosie's pain is intensified by the knowledge that she has a fifty percent chance of inheriting the crippling illness. Only when Rosie tells her mother's best friend, "Aunt Sarah" that she is going to be tested for the disease does Sarah, a midwife, reveal that Trudie wasn't Rosie's mother after all. Rosie was swapped at birth with a sickly baby who was certain to die.