In "A Thousand Splendid Suns," what did the author mean by the quote, "If it is a girl's name Lailah already said it?"

The quote, "...if it's a girl, Laila has already named her" (Chapter 51), is the last line of the story.  Laila is pregnant, and is remembering a naming game her family had played the night before.  The game only involved boys' names, because if the baby is a girl, Laila already knows what she will name it - "Mariam".

By saving her from their brutish husband Rasheesh, then taking full responsibility for his death and in effect sacrificing herself to the Taliban, Mariam has given Laila and and Tariq the chance to start anew and live life without being fugitives.  Laila remembers Mariam, who had so few to love her in her own life, as "a rock in a riverbed...her grace not sullied but shaped by the turbulence that washe(d) over her...(with) something deep in her core, that neither Rasheed nor the Taliban (could) break" (Chapter 50).  Filled with love and gratitude, Laila will always carry Mariam in her heart, "where she (will shine) with the bursting radiance of a thousand suns" (Chapter 51).  There is no doubt that if her baby is a girl, Laila will give her Mariam's name.