How Do I Trace Illegitimate Ancestor Further?

I have a 3rd Great Grandmother born illegitimately in 1851, her parents are listed but I don't think they married as far as I can tell, I can't find any record of it at least. My question is how do I find her parents parents if I can't find their marriage records which is usually how I could figure something out? I will put the birth records and marriage record of my ancestor below. https://www.digitalarkivet.no/en/view/327/pv00000004451895 https://www.digitalarkivet.no/en/view/255/pd00000005166083 Thank you everyone in advance for any advice.