What is the single best measurement of gestational age in the third trimester?

The best measurement of gestational age in the third trimester is femur length. This measurement is particularly valuable because it is less susceptible to the variations that can occur in head size or body proportions, which may not accurately reflect gestational age due to factors like fetal growth disorders or genetic variations.

As pregnancy progresses into the third trimester, the femur length becomes a more reliable marker for assessing fetal age, primarily because the long bones are proportionate and grow steadily. This reliability is critical for making informed clinical decisions, particularly regarding monitoring fetal growth and estimating delivery dates.

In contrast, other measurements such as bi-parietal diameter and head circumference are subject to influences like fetal positioning and head shape, while abdominal circumference can be affected by factors such as fetal fat deposition and maternal body composition. Therefore, femur length stands out as a consistent and effective metric for estimating gestational age during this later stage of pregnancy.