.0% and 13.5%, respectively). The observer and measurement explained a small proportion of variance in both trimesters (1.4% and 0.4%, respectively in the 1st trimester, and 3.3% and 0%, respectively in the 2nd trimester). The residual variances were 10.6% and 11.2%, and the standard errors of measurement were 1.78 mm and 1.82 mm for the first and second trimesters, respectively.