What is the least helpful ultrasound parameter for dating in the third trimester?

In the context of third-trimester ultrasound dating, biparietal diameter (BPD) is considered the least helpful parameter. This is primarily due to the fact that during the later stages of pregnancy, fetal growth patterns can vary significantly between individuals, influenced by factors such as maternal nutrition, health status, and fetal genetic factors. The BPD can be affected by the fetal head shape and may not accurately reflect fetal size as gestation progresses.

In contrast, parameters such as femur length (FL), abdominal circumference (AC), and head circumference (HC) tend to provide more reliable estimates of gestational age during the third trimester. These measurements can better reflect overall fetal growth and are less prone to variation than the BPD. The AC, for example, is particularly useful as it accounts for the volume of the fetus and can indicate growth trends more accurately, whereas HC provides insight into brain development and cranial growth, both of which closely correlate with gestational age.

Thus, while BPD can still provide some information, its utility wanes in the third trimester compared to the other measurements, making it the least helpful for precise dating at this stage.