The birds, the bees and orphanages?

I have been getting a lot of questions from Cameron about "where babies come from". It started last year when a lot of his friends were having baby brothers and sisters. He asked "Mommy, did so and so's sister come from China". I just said, No, she grew in her mommy's tummy. "oh" he said and that seemed to be enough. Then he would say things like " did you come here Before I came, when I was in the orphanage?"then I had to explain how he grew inside my tummy. Then came the usual questions about how he got in there, etc...I used simple terms like a woman has an egg and the man fertilizes it and it makes a baby grow...But today he put two and two together and said "so Gracie didn't grow in YOUR tummy, but she grew in someone's tummy, right? (smart boy!) I said, yes, she grew in her birth mommy's tummy. Then he went on to ask why her birth mommy put her in the orphanage and why didn't I put him in the orphanage?? I had to think about that one...I am just trying to get to school on time here, OK??? So I said that Gracie's birth mommy couldn't take care of her, so she brought her to the orphanage so I could become her mommy. I am not sure if I said it the right way, but it was the best I could do at the time and she was in the back seat listening to all this, by the way! So, if anyone has any suggestions on how to better address, please advise!!