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

Step 1: Understanding the Question:
Ultrasound can estimate fetal gestational age by measuring different fetal structures, and the most accurate parameter changes as pregnancy progresses. We need the parameter that is used to best estimate age specifically in the third trimester.

Step 2: Key Formula or Approach:
Dating accuracy is highest with crown rump length in the first trimester. From the second trimester onward, head measurements such as the biparietal diameter (BPD), taken at the level of the thalami, are the traditional workhorse measurement for fetal age, and BPD remains one of the standard core parameters reported at every scan through the third trimester alongside head circumference, abdominal circumference, and femur length.

Step 3: Detailed Explanation:
Femur length: reflects long bone growth and is useful, especially when combined with other parameters, but on its own it is not the traditional first choice parameter quoted for third trimester dating.
Biparietal diameter: this transverse skull measurement has long been the standard reference parameter used across pregnancy for fetal age estimation, including into the third trimester, which is why it remains the answer expected here.
Abdominal circumference: this reflects fetal size and nutrition more than skeletal age, and is more useful for judging growth (small or large for dates) than for pure age estimation.
Inter-ocular distance: this is a specialised measurement used mainly to screen for conditions like hypotelorism or hypertelorism, not a routine parameter for dating.

Step 4: Final Answer:
Among the options, the biparietal diameter remains the parameter classically used for fetal age estimation by ultrasound in the third trimester.
\[ \boxed{\text{Biparietal diameter}} \]