In the book, Wanda explains that Motherhood was treasured, valued, and esteemed above all else, so if the couple did not want to give up their baby as a host for a new soul, they would not be forced to. I think, this means too, that all souls would respect the couple and not coerce or think ill of them keeping a human baby. It is interesting to think how with souls only some have a capacity for Motherhood (Wanda is one of them) and that it is also a death sentence should the soul decide to have more souls. As "humans", all souls now have an ability to experience Parenthood--not just the Mothers, but Fathers as well. It gives them new emotions, new horizons, new dreams, and a new future to pursue that they couldn't anywhere else. And, I know that personally (in hindsight), there are little signs of a baby's personality that show up before they ever make their appearance (ex: during my pregnancy with my eldest, I never got a moment's peace, especially at night--she was so active; but with my second, she was active all day and very quiet at night---they are both still that way and now they're 6 and 3). Anyhoo, I guess my point is the souls would have bonded with their child before he was born and wouldn't have wanted to lose him to another and completely different personality after he was born. Perhaps too, they were considering the excitement that would come with discovering how their little one's life would unfold and the unique aspects of his little personality.
This scene is definitely a hopeful one...though I think it is slightly clouded. How will the souls be able to continue on earth if they don't continuously take over humans? Also, too, what happens if some of the human children grow up to resent souls taking over their race?
I bet Stephenie Meyer has this in her head.