AHA! So it was a process of elimination. Margrete's son could have been born at the beginning of January 1819....I still wrote them both down...Now there's another daughter? Petronille must have stayed in Norway? I can see your reasoning though about the names.
I also found 2 Petronelle Pedersdtrs getting married, one in 1863 and the other in 1868....I thought the 1868 made more sense. because the age of marriage was 18 1/2, but I couldn't find either her or Johannes Martinsen in the 1900 census....