End of First Trimester?

I am just as confused as Courtney is when it comes to determining the end of the first trimester....(I don't remember this confusion when I was pregnant...but then again, pregnancy was not counted until conception....now there's an additional two weeks before conception....how confusing!)

This is what I found on the internet:

One way is to look at development. This way, 2nd trimester starts at 12 weeks and 3rd starts at 27 weeks.

Another is by gestation - this simply divides the 40 weeks of pregnancy into three equal stages, which gives 2nd trimester starting at 13 weeks and 3 days. 3rd trimester starts at 26 weeks and 5 days this way.

The third is by conception - take the 38 weeks of pregnancy that occur after conception and divide that into three equal stages. This way gives 2nd trimester starting at 14 weeks and 5 days, while 3rd trimester starts at 27 weeks and 3 days.

The third trimester dates all seem to fall pretty close together at around 27 weeks, which is why I guess the conflict doesn't seem to arise there, but there are vastly different dates for second trimester. Averaging them out gives something around 13 weeks, but information will vary as to which method was actually used to calculate them.

I think we will go with the 13w3d as the start of Courtney's 2nd trimester, which will be on Sept. 28, 2009.....I keep saying extra prayers for little baby Barnett!