Which ultrasonographic measurement maintains closest correlation with gestational age in the second trimester?

• A. Biparietal diameter (BPD)
• B. Crown-rump length
• C. Head circumference
• D. Femur length

Answer: (A)

The main idea is that certain fetal measurements correlate more tightly with gestational age than others during the second trimester. The biparietal diameter tracks steady skull width growth with a linear relationship to gestational age in mid-pregnancy, making it the most reliable single metric for dating in this period. Head circumference also reflects skull size but can be affected by head shape, adding variability. Crown-rump length is a first-trimester measure, not used for dating in the second trimester. Femur length varies with fetal growth patterns and bone maturation, which can introduce more error for dating in this window. So, the measurement that stays most closely tied to gestational age in the second trimester is biparietal diameter.