Regarding why one child was allowed to go to term- I think it's possible he beat her in an attempt to end it, and left for a period (perhaps without feeding her well) and later found out he wasn't successful when she started to show more.
He may have taken it as some sort of sign and decided to let her carry to term simply bc the child survived his typical "method." Some people are incredibly superstitious- good and bad.for all we know he had a dream that he should let one live. It's impossible to know.
I don't know this at all- it's just one possibility that could explain why one child was allowed to go to term if others were in fact now allowed to.
I do think he may have favored/trusted Amanda more- but I still don't think he wanted a bunch of kids to hide, either.