The simple answer..."the Force is strong in this one." Anakin/Vader knows his strength from his arrogant youth and knows his weaknesses from his 15+ years of servitude, (additionally his regrets.)
As for Padme, she did gasp after he stopped choking her. But regardless, I believe he knew she died but was unaware she gave (premature?) birth of her children prior to that. Clearly her death was known by Yoda and his interest and (Jimmy Smits) was to not hide it.
As to whether the Force should be strong enough to tell if her body still held his unborn children...I am not strong enough to tell.