Gestational age ultrasound: crown-rump length and biparietal diameter

Accurate gestational age ultrasound depends on obtaining the correct image and making precise measurements. In this lesson, learn how to acquire the appropriate ultrasound views and accurately measure crown-rump length and biparietal diameter so you can confidently estimate gestational age during early pregnancy ultrasound.

In this lesson from our First Trimester Ultrasound Power Hour course you'll learn how to:

• Perform gestational age ultrasound in early pregnancy
• Select crown-rump length or biparietal diameter based on the stage of pregnancy
• Identify the correct view for crown-rump length measurement
• Estimate first-trimester gestational age using crown-rump length
• Obtain the correct fetal skull view for biparietal diameter measurement
• Estimate second-trimester gestational age using biparietal diameter

Gestational age ultrasound

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Once you've identified a definitive IUP [intrauterine pregnancy], you can further assess for gestational age by ultrasound. There are a few different techniques you can use. This will depend on the estimated gestational age. Early in pregnancy, in the first trimester, you will determine the gestational age using the crown-rump length. In your second trimester, you will measure the biparietal diameter of your fetus skull to estimate gestational age. Late in pregnancy, you can use the femur length to estimate gestational age.

First trimester ultrasound for gestational age

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In this lesson, we will go over the first two techniques which you may use in an early pregnancy evaluation. Here we're seeing an image of a transabdominal pelvis study identifying a fetus within a gestational sac. Once you identify the fetus, fan through the image until you see the fetus in the longest cross-section. Freeze your image here.

Measuring crown-rump length in first trimester ultrasound

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Next, take out your calipers and measure from the crown, or the top of the head, to the rump, which is essentially the bottom. If you are in the OB setting or abdominal setting of your ultrasound machine, you should be able to input this measurement to generate a gestational age.

Using biparietal diameter ultrasound in the second trimester

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In the second trimester, you'll estimate gestational age on ultrasound using the biparietal diameter. To obtain that measurement, scan through the skull of your fetus. You want to hold your probe as perpendicular as you can to the skull of your fetus.

How to measure biparietal diameter on ultrasound

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Scan through it until you see symmetric hemispheres, then freeze your image, and using your calipers, you want to measure it from the outer wall to the inner wall of your fetus skull. Inputting this measurement into your ultrasound machine should generate your estimated gestational age.