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.
"The Gestation method -- take the 40 weeks of gestation and divide by three. Here the second trimester begins at 13w 3d, and the third at 26w 6d.
"The Conception 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.