How was David separated From his mother?

In the last chapter David is reunited with his mother. When he finally reaches the address, David is so tired he can barely walk. Narration describes how he felt that "if his happiest dreams came true, he could go on living: if not, this was the end." He picks up his bundle and rings the doorbell. A woman answers, and David knows immediately that it is the woman from Sophie's photograph. He begins to introduce himself, but the woman interrupts him. "The woman looked into his face," narration states, "and said clearly and distinctly 'David ... my son David.'"