Yes, in theory with a natural BFP your CL cyst is supposed to produce enough e2 and p4 to sustain your pg until the placenta develops. 10 is usually the acceptable level of p4 in early pg (but some Drs like to see 20+) and slowly rising to 30-50 and then leveling off. So a level of 5 is not enough and your Dr should have put you on p4 support. Since you are over 40 your egg quality might be starting to decline, but this just means that all of the things that are supposed to happen on their own might not be happening, which includes forming a functional CL that sustains for 10 wk. So I would definitely get both levels tested on your next BFP right away.