The first trimester is the first 13 weeks based on your LMP. At that point your baby is 11 weeks old! This is when the risk of miscarriage goes down like 80% or more!
This is what I have been reading anyway!
i believe the second trimester starts when you are 13 weeks..i go to ask.com and type in pregnancy week by week and the first one that comes up, i think it says pregnancy week by week sponsored by storknet, or something close to that..and it tells u how the baby grows each week and your body changes..and by that website, it says 2nd trimester is at 13 weeks!