How is gestational age determined in the second trimester?
The most accurate estimation of gestational age is in the first trimester based on embryonic crown-rump length. In the second trimester (after 14 weeks gestation), dating is based on a combination of fetal measurements including biparietal diameter (BPD) (measured in the transverse plane at the level of the thalami from the outer edge of the proximal skull to the inner edge of the distal skull), head circumference (HC) (also measured at the level of the thalami), abdominal circumference (AC) (measured at the level of the fetal stomach and junction of the umbilical vein and portal venous sinus), and femoral diaphysis length (FL).
These measurements are also used in calculations for estimation of fetal weight. A head-to-abdominal circumference ratio can also be calculated to assess appropriate fetal growth. The HC is normally larger than the AC in the second and early third trimesters, with a reversal of this ratio at term.