Objective To explore the accuracy of four fetal estimation formulas generated by Hadlock with or without head circumference parameter in fetus with small head size.

Methods From February 2014 to August 2020, 328 singleton deliveries who gave birth at the Obstetrics and Gynecology Hospital of Fudan University were selected. Birth weights were compared with estimated fetal weights calculated by four Hadlock formulas. Percentage error, absolute percentage error, and the proportion of weight estimations falling within ±15% error interval were also calculated. Intraclass correlation coefficients and Bland-Altman analysis were used to assess the concordance between estimated fetal weights and birth weights.

Results The mean birth weight was (2 084.4±778.1) g. The mean absolute percentage error ranged (8.9±8.9)% to (10.1±9.7)%. The proportion of weight estimations falling within ±15% error interval ranged between 80.2% and 83.5%. These differences were statistically insignificant. Using Bland-Altman analysis, the mean differences were all tiny, fluctuated between 6.5 g and 73 g. The intraclass correlation coefficients generated by four formulae were superior (ICC>0.75).

Conclusions The accuracy and concordance difference between estimated fetal weight and birth weight among four formulae were minor in fetuses with small head circumference.