Fetal Measurements, Tips and Tricks

Summary

This lecture provides a detailed overview of fetal biometry and its importance in determining gestational age using ultrasound. Key measurements such as the mean sac diameter, crown-rump length, biparietal diameter, head circumference, abdominal circumference, and femur length are discussed. The script explains how to correctly measure each parameter, common mistakes to avoid, and the significance of each measurement for accurately assessing fetal growth and development. Emphasis is placed on achieving accurate imaging techniques and caliper placement to ensure precise results.

Takeaways

• 😀 Fetal biometry refers to the measurement of fetal anatomy through ultrasound to assess gestational age and fetal growth.
• 😀 In the first trimester, the most common measurements are mean gestational sac diameter and crown rump length.
• 😀 For crown rump length measurement, the fetus should be in a mid-sagittal section and in a neutral, horizontal position.
• 😀 The mean sac diameter can be used to determine gestational age in early pregnancy, especially when the fetal pole is not visible.
• 😀 Crown rump length is the most accurate method for determining gestational age between 8 and 12 weeks.
• 😀 Biparietal diameter and head circumference are key measurements for assessing fetal head growth from 12 weeks onward.
• 😀 The biparietal diameter measurement should be taken at the widest part of the fetal skull, from outer to outer edges.
• 😀 Head circumference, measured using ultrasound's ellipse feature, provides a more accurate estimate of gestational age than biparietal diameter.
• 😀 Femur length is a reliable measurement for determining gestational age and is useful when other measurements like biparietal diameter cannot be obtained.
• 😀 Abdominal circumference is not used to determine gestational age, but it is important for assessing intrauterine growth restriction or fetal macrosomia.
• 😀 Correct placement of calipers, appropriate magnification, and the correct fetal position are essential for accurate fetal measurements.

Q & A

What is fetal biometry?

Fetal biometry refers to the measurement of the anatomical segments of the fetus by ultrasound, typically used to assess fetal development and estimate gestational age.

What are the key measurements taken in the first trimester for fetal biometry?

In the first trimester, the key measurements include the mean gestational sac diameter and crown rump length.

How is the mean sac diameter (MSD) measured?

To measure MSD, obtain longitudinal and transverse views of the gestational sac. Measure the length, width, and height of the anechoic area, excluding the echogenic rim, and average these dimensions.

How do you calculate gestational age if the mean sac diameter is out of range?

If the MSD is out of range, use the formula: MSD in millimeters + 30 = gestational age in days. For example, if the MSD is 9 mm, the gestational age is 9 + 30 = 39 days, which equals about 5 weeks and 4 days.

Why is crown rump length considered the most accurate method for determining gestational age in the first trimester?

Crown rump length is the most accurate for gestational age determination because it reflects the longest embryo length, excluding limbs and yolk sac, with an accuracy of 3 to 5 days when measured between weeks 8 and 12.

What are the key points to ensure accurate crown rump length measurement?

Ensure the fetus is in a neutral position, the mid-sagittal section is used, and the image is sufficiently magnified. Calipers should be placed at the outer borders of the head and rump, not the inner borders.

How does biparietal diameter (BPD) contribute to estimating gestational age?

BPD is measured at the widest part of the fetal skull, and it was the first ultrasound parameter used for assessing gestational age. The optimal time for this measurement is between weeks 12 and 24, providing a reliable estimate of gestational age.

How is head circumference measured using ultrasound?

Head circumference is measured using the ellipse facility on the ultrasound machine. The ellipse should be placed around the outer perimeter of the skull at the same level as BPD, and it is preferred over BPD for assessing gestational age due to its reduced sensitivity to head shape variations.

What is the role of femur length in determining gestational age?

Femur length is another key measurement, particularly when fetal position prevents proper measurement of BPD or head circumference. It is most accurate as a gestational age predictor when measured from 12 weeks to term, although its accuracy decreases as gestational age increases.

Why should the greater trochanter not be included when measuring femur length?

Including the greater trochanter in the femur length measurement results in overestimation, which can lead to an incorrect estimate of gestational age.

How is abdominal circumference used in fetal measurements?

Abdominal circumference is not used for determining gestational age but is important for assessing intrauterine growth restriction and fetal macrosomia. It is measured using the ellipse feature of the ultrasound machine, ensuring the section is circular, and the stomach bubble is visible.