What is considered the least useful measurement for differentiating between fetal growth restriction (FGR) and inaccurate dating?

The measurement considered the least useful for differentiating between fetal growth restriction (FGR) and inaccurate dating is the abdominal circumference (AC). This is primarily because AC is significantly influenced by both fetal size and gestational age.

In cases of FGR, the fetal head and body may still develop proportionately for specific gestational ages, but the abdominal circumference may be more greatly affected by nutritional status and placental function. Therefore, a smaller AC might not necessarily indicate true growth restriction but could instead reflect inaccuracies in gestational dating or abnormalities in fetal growth patterns.

In contrast, the transcerebellar diameter (also known for evaluating central nervous system development), head circumference (HC), and biparietal diameter (BPD) provide more reliable measures concerning gestational age. They typically show more consistent correlations with gestational age and are less prone to variation due to the factors that affect abdominal size. Thus, utilizing these cranial measurements can offer a clearer distinction between true FGR and possible dating issues, making the AC the least reliable metric in this context.