Fetal biometry assessment with Intergrowth 21st's and Salomon's equations in rural Burkina Faso

Abstract

Background: Ultrasound scanning during the 2nd or the 3rd trimester of pregnancy for fetal size disturbances screening is heavily dependent of the choice of the reference chart. This study aimed to assess the agreement of Salomon and the Intergrowth 21st equations in evaluating fetal biometric measurements in a rural area of Burkina Faso, and to measure the effect of changing a reference chart.

Methods: Data collected in Nazoanga, Burkina Faso, between October 2010 and October 2012, during a clinical trial evaluating the safety and efficacy of several antimalarial treatments in pregnant women were analyzed. We included singleton pregnancies at 16-36 weeks gestation as determined by ultrasound measurements of fetal bi-parietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL). Expected mean and standard deviation at a given gestational age was computed using equations from Salomon references and using Intergrowth 21st standard. Then, z-scores were calculated and used subsequently to compare Salomon references with Intergrowth 21st standards.

Results: The analysis included 276 singleton pregnancies. Agreement was poor except for HC: mean difference - 0.01, limits of agreement - 0.60 and 0.59. When AC was used as a surrogate of fetal size, switching from the reference of Salomon to the standards of Intergrowth 21st increased ten times the proportion of fetuses above the 90th percentile: 2.9 and 31.2%, respectively. Mean differences were larger in the third trimester than in the second trimester. However, agreement remained good for HC in both trimesters. Difference in the proportion of AC measurements above the 90th percentile using Salomon and Intergrowth 21st equations was greater in the second trimester (2.6 and 36.3%, respectively) than in the third trimester (3.5 and 19.8%, respectively). The greatest difference between the two charts was observed in the number of FL measurements classified as large in the second trimester (6.8 and 54.2%, using Salomon and Intergrowth 21st equations, respectively).

Conclusion: The agreement between Intergrowth 21st and Salomon equations is poor apart from HC. This would imply different clinical decision regarding the management of the pregnancy.

Keywords: Biometry; Burkina Faso; References; Standards.

Fig. 1

Fetal biometry measurements by gestational age. a: Biparietal diameter. b: Abdominal circumference. c: Head circumference. d: Femur length. Intergrowth 21: International fetal and newborn growth consortium for the twenty-first century. The numbers at the right of the boxplots represented from the bottom to the top: the minimum without outliers, the first quartile, the median, the third quartile and the maximum without outliers

Fig. 2

Number of standard deviations from the mean using Salomon or Intergrowth 21 eqs. a: Biparietal diameter. b: Abdominal circumference. c: Head circumference. d: Femur length. Intergrowth 21: International fetal and newborn growth consortium for the twenty-first century. Dashed horizontal and vertical grey lines referred to a z score of − 1.282 corresponding to the 10th centile. Solid horizontal and vertical grey lines referred to a z score of 1.282 corresponding to the 90th centile. The black oblique line is the perfect concordance line where the z-scores from Intergrowth 21st and Salomon equations are equal

Fig. 3

Agreement between z scores using Salomon or Intergrowth 21 equations. a: Head circumference. b: Femur length. c: Biparietal diameter. d: Abdominal circumference. Intergrowth 21: International fetal and newborn growth consortium for the twenty-first century. Dashed horizontal grey lower line represents the lower limit of agreement between the z-scores from Intergrowth 21st and Salomon equations. Solid horizontal grey line represents the mean difference between the z-scores from Intergrowth 21st and Salomon equations. Dashed horizontal grey upper line represents the upper limit of agreement between the z-scores from Intergrowth 21st and Salomon equations

Fig. 4

Regression of Intergrowth 21 z-scores with Salomon z-scores. a: Biparietal diameter. b: Abdominal circumference. c: Head circumference. d: Femur length. Intergrowth 21: International fetal and newborn growth consortium for the twenty-first century. Dashed oblique black line represents the perfect agreement between the z-scores from Intergrowth 21st and Salomon equations. Solid oblique grey line represents the linear regression fitted line