Revisiting the Hadlock 1991 population reference for estimated fetal weight

The most widely used population reference standard for fetal growth assessment in the United States is the Hadlock 1991 population reference. However, we detected discrepancies in the Hadlock 1991 publication between its stated methodology and its presentation of critical point values. This study investigates these discrepancies and their potential effects on clinical practice and research.

The Hadlock 1991 table describing a fetal weight standard was recalculated from the Hadlock 1991 formula as printed in the body of the text. A difference was observed between the equation results and the table as published in the 1991 manuscript. These 2 methods for assessing estimated fetal weight percentiles were retrospectively applied to a cohort of all singleton growth ultrasounds performed at our institution since 2015. We also collected a sample of manuscripts in the literature that reference Hadlock 1991 to determine if methodology varied among previously published reports.

The Hadlock 1991 equation does not produce the same critical point values for estimated fetal weight as the table presented in the manuscript. In our database of 176,060 ultrasound encounters for growth in singleton fetuses (78,660 patients), the Hadlock table, as compared to the Hadlock equation, would have resulted in underdiagnosis of fetal growth restriction in 5.1% (4009/78,660) of patients. Manuscripts that compared Hadlock 1991 to other growth standards tended to favor Hadlock 1991 if the equation was used, and disfavor it if the table was used.

The Hadlock 1991 regression equation presented in the manuscript and the table are not the same. The table underdiagnoses fetal growth restriction and, potentially, impacts clinical intervention for fetuses at risk for growth abnormality. Inconsistencies in previous studies on the detection of fetal growth restriction using Hadlock 1991 may be attributed to these discrepancies. If increased detection of fetal growth restriction is desired, then Interpolation of the Hadlock 1991 table should be avoided and the equation preferred instead. Additional validation of these findings is required to ascertain clinical significance.