Hope this helps! (Quoted off another site)
There are basically three ways of dividing up a pregnancy into the three trimesters; they are by:
Development, Gestation, Conception. They give different dates for when the second trimester begins and for when the third trimester begins. Your health care provider might prefer to use the Development method, while another Midwife or Doctor may use the Gestation method.
I am sure they do that just to confuse pregnant women.
by Development:
This uses actual developmental stages to divide up a pregnancy. From LMP to 12 weeks the embryo develops all the major organs and becomes a fetus. From 12 weeks to 27 weeks the fetus continues developing and reaches viability. From 27 weeks on the fetus finishes development and prepares for delivery. Here the second trimester begins at 12w 0d and the third at 27w 0d.
by Gestation:
With this method you take the 40 weeks of gestation and divide by three. Here the second trimester begins at 13w 3d and the third at 26w 6d.
by Conception:
This method is where you take the 38 weeks of post conception development, divide by three, and add two weeks. Here the second trimester begins at 14w 5d and the third at 27w 3d.
With all of the other apps and things and experience I have its 12 weeks for each trimester
I've just heard that there are 14 weeks in a trimester!! Which if you think about pregnancy might be 42 weeks, it doestooes sense. That could be the confusion! I always thought 12 weeks is the end of the first trimester too!
Lol I know I just thought it was funny that this app says were still in the first trimester:) this is our fourth baby how about you what #
Im 13 weeks today your bow in second trimester