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.
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.
Conception:
This method is where you take the 38 weeks of post conception development, divide by three, and add two weeks.