Question

Best parameter for estimation of fetal age by ultrasound in the third trimester is:

• A. Femur length
• B. Biparietal diameter
• C. Abdominal circumference
• D. Inter-ocular distance

Hint:

BPD is the traditional standard measurement used for fetal age estimation on ultrasound through the third trimester.

Answer 1

The Correct Option is B

Ultrasound dating relies on measuring fixed fetal structures and comparing them against known growth charts. The best single parameter changes across the three trimesters. Let's look at each option for the third trimester specifically.

1. Femur length: useful and commonly combined with head measurements for a composite estimate, and some texts note it holds up reasonably well later in pregnancy since it is less affected by head shape changes, but it is not the parameter classically named as the standard for this stage in most exam texts.
2. Biparietal diameter: measured across the fetal skull at the level of the thalami, this has been the traditional core parameter for fetal age on ultrasound since the second trimester and continues to be used through the third trimester as one of the standard dating measurements.
3. Abdominal circumference: this measurement tracks fetal soft tissue and liver size, so it is more sensitive to nutritional status and growth restriction than to fixed skeletal age, making it a growth parameter rather than a dating parameter.
4. Inter-ocular distance: this is a niche measurement checked when the face or orbits look abnormal on scan, not a routine tool for estimating gestational age.

Let's summarize:

• Biparietal diameter is the traditional standard parameter used for fetal age across pregnancy, including the third trimester.
• Abdominal circumference is better for judging growth than age, and inter-ocular distance is a specialised, not routine, measurement.

So the best parameter here is the biparietal diameter.