197 days to go...

We have 197 days to go, based on our due date. I'm a bit confused by the use of weeks to count how far along one is. The online calendar we were using looks says that we just begun with week 12. I think that this is wrong. My wife and I looked at a calendar and counted that we are starting on the 13th week since her last period started. We purchased a pregnancy journal which says that in a couple of days, we'll be on week 11 (based on conception) or LNMP week 13 (based on start of last cycle). Assuming that we are done with the first 12 weeks, that'd mean we are starting on our second tri-mester. However, our journal shows the second tri-mester starting at the beginning of week 13 or 15 (LNMP) with 181 days to go. So, today, I am not sure if we are beginning our 2nd trimester or not. Regardless, the baby will come when it is ready.