anonymrs:
As far as I know, oomis is correct:
If there was a miscarriage in the first pregnancy, there is no requirement for a pidyon on the next baby born if it is male, since a pregnancy has already exited the womb previously, even though it was not viable.
I never heard about a pidyon being required for a second child if the first was by C-section.
Also, I believe, but may be wrong, a pidyon is not required if the bechor was delivered by C-section.