Fetal biometry in the 2^nd and 3^rd trimesters – simple or actually complicated?

Prenatal ultrasound is essential for identifying abnormal fetal growth. Since the introduction of ultrasound in prenatal care, attempts have been made to help improve the detection of small-for-gestational age (SGA) fetuses, large-for-gestational age (LGA) fetuses, and cases of intrauterine growth restriction (IUGR). Abnormal fetal growth can increase the risk for obstetric complications and can also be the result of a fetal disease requiring further prenatal or postnatal diagnostic testing.

An essential requirement for the correct interpretation of measured values in fetuses is the exact determination of the gestational age in the 1^st trimester, which is one of the main purposes of the 1^st-trimester ultrasound examination.

Correct selection of the measurement planes needed for biometry and exact placement of the measurement points are some of the first things taught and practiced in ultrasound education.

Fetal biometry is primarily used to calculate fetal weight, even though the individual biometric parameters are also used in diagnostic prenatal ultrasound. By comparing measured values to corresponding reference values, it is possible to determine whether the fetal mass and weight correspond to the gestational age. Serial measurements provide information about fetal growth.

Even though fetal biometry is part of almost every prenatal ultrasound examination and the correct measurement planes for head circumference, abdominal circumference, and femur length are clearly defined and globally standardized [1], there are various aspects that are not uniformly defined or are handled differently.

Caliper placement

The placement of the calipers for measurement points represents a source of error which is not insignificant. Indeed, differences can be seen in the placement of the calipers for the creation of various reference curves. This plays a significant role, particularly for the biometry of the fetal head. Placing the calipers in different positions, i. e., outer edge, proximal outer edge, and distal inner edge of the skull, or the decision whether or not to include the skin in the measurement has been handled differently in various reference curves [2] [3] ([Fig. 1]). This means that the examiner must be aware of the caliper positioning applied to create the reference curve in order to generate reproducible measurement results. When measuring the occipitofrontal diameter, it is necessary to ensure that reverberation artifacts do not result in false large measurements ([Fig. 1]) [3].

Fig. 1 Yellow arrows: possible caliper placement depending on reference curve. Red arrows: incorrect caliper placement (artifacts).

Various methods are also used to determine the head circumference and the abdominal circumference. The circumference can be determined with a formula, using longitudinal and transverse measurements [4] or the circle/ellipse fitting function of the ultrasound device can be used to measure the circumference of the relevant plane.

Again, it is important, to apply the same method that was used to create the reference curve.

A further source of error in biometry that should not be underestimated is the tendency of examiner to generate the expected measurement value which can result in an one-sided measurement error. By filming the eye movements of examiners it could be shown that caliper placement was adjusted in over 90 % of cases in which the measured value displayed on the ultrasound image did not meet the examiner’s expectations [5].

 

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Calculating the estimated fetal weight

The estimated fetal weight is essential for clinical obstetrics and for communication with the pediatrician. In addition to the numerous reference curves for the individual biometry values, there is also a wide range of formulas for calculating the estimated fetal weight. The ideal weight formula should have a minimal error of estimation for every gestational age – regardless of the actual fetal weight. However, this is relatively difficult to achieve. Therefore, an attempt has been made to reduce the error of estimation in large-for-gestational age and small-for-gestational age fetuses using specific formulas for extreme fetal weights. However, since the actual fetal weight is not known, selecting the optimal formula for estimation based on fetal weight is not possible. The formula of Hadlock et al. from 1985 [6] has proven to have an acceptable error of estimation in all fetal weight classes [7]. This formula uses the head circumference, abdominal circumference, and femur length for calculation.

 

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Reference curves

Different approaches for reference curves exist. General reference curves that are valid for all populations like the Intergrowth21 Project or the WHO curves, population-based reference curves, i. e., reference curves created with data from the population in which the curve is intended to be used, and curves with inclusion of maternal variables, such as body mass index and ethnicity.

General reference curves that are valid for all populations like the Intergrowth21 Project do not seem to be ideally suited to all populations [8]. Population-based reference curves are probably better for ensuring correct grouping of measured values into percentiles and identifying deviations from the norm. Thus, a comparison of different growth curves to detect placental-related fetal growth restriction was able to show that the detection rate and false-positive rates were highly dependent on the reference curve being used, and a significant deviation was seen particularly in the case of the Intergrowth21 Project [9]. Growth curves taking anthropometric and ethnic parameters of the mother into account do not seem to have significant advantages – particularly with regard to detecting small-for-gestational-age fetuses [10]. Therefore, reference values based on the population to be examined should be used when possible.

 

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Summary

Requirements for correct biometry:

• Correct determination of gestational age

• Exact selection of measurement planes

• Caliper placement in accordance with the reference curves being used

• Calculation of the estimated fetal weight using the “Hadlock 3” formula

• Reference curves based on the population being examined

 

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Prof. Dr. med. Leonhard Schäffer

PD Dr. med. Tilo Burkhardt

Conflict of Interest

The authors declare that they have no conflict of interest.

• References

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Publication History

Article published online:
06 June 2025