My last cycle was oct 6th my Doc said I'm 4 week how is that so when conception was about the 20th of oct

my last cycle was oct 6th my Doc said im 4 wk how is that so when conception was abt the 20th of oct

The 4 weeks is going by the gestational age... taken from the start of your last period. Obviously most women are not actually pregnant at that point and this is partly why you have several weeks at the end of pregnancy that would be considered a normal time frame for giving birth.

At some point you will likely have a scan done and this can alter your due date a bit as it will usually give you a bit more accuracy as to the fertilizational age... from when you most likely conceived.