Accuracy of fetal weight estimation by modified Johnson method compared with ultrasound at Suratthani Hospital
Keywords:
Accuracy, Fetal weight estimation, Uterine fundus measurement, Ultrasound, Birth weightAbstract
Background: There are several methods of estimating fetal weight, such as abdominal examination by Leopold maneuver, measuring the uterine fundus by a tape, or ultrasound. Accuracy of fetal weight estimation affects the delivery planning. Measuring the uterine fundus combined with fetal head level assessment by per vaginal examination (station), and calculating with modified Johnson’s formula is one method that is simple and has no cost. Therefore, our team aimed to compare its accuracy with ultrasound examinations by interns at Surat Thani Hospital.
Objectives: To evaluate the accuracy of fetal weight estimation between uterine fundus measurement using the modified Johnson method and ultrasound examination by interns in term pregnant women who delivered at Suratthani Hospital.
Method: A total of 391 singleton pregnant women who delivered between November 1, 2024 and May 31, 2025, were performed fetal weight estimation using the modified Johnson method and ultrasound examination by interns. An accuracy of the two methods with actual birth weight were compared.
Results: There was no statistically significant difference between the actual birth weight and the estimated birth weight using either the Modified Johnson’s formula (P = 0.811) or ultrasound (P = 0.467). The differences between the actual birth weight and the estimated values obtained by ultrasound and the Modified Johnson’s formula were 230.2 ± 200.4 grams and 281.6 ± 227.4 grams, respectively. Ultrasound showed the lowest mean absolute percentage error compared with the Modified Johnson’s formula. However, when considering accuracy within 10% of the actual birth weight, the Modified Johnson’s formula demonstrated greater accuracy than ultrasound (P < 0.001).
Conclusion: Estimated fetal weight using modified Johnson method was more accurate than ultrasound performed by interns.
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