So which one is it really (length of trimester clarification)?

One article says the first trimester ends after 12 weeks and the next says 13 weeks...I know it is not a big deal...I am just looking forward to being "done with the first trimester". What do doctors really say? When am I hitting my safer zone? I am currently 11 + weeks...baby is growing well (had 3 ultrasounds due to prevous miscarriages) and have heard heartbeat on rented doppler at home several times (yesterday included) I feel good about the baby so far I just love reurance!