Estimation of Fetal Birth Weight at Delivery: Which is More Accurate? Comparison of Ultrasound Hadlock Versus Clinical Dare’s Methods across Maternal and Fetal Weight

Background: Accurate fetal birth weight estimation is crucial for a smooth, complication-free birthing experience. However, the rising rates of maternal obesity present significant challenges for obstetricians, frequently leading to over or underestimation that can have serious consequences.

Methods: This prospective cohort study involved 154 singleton pregnancies within a week of delivery. Fetal weight was estimated using both Dare’s clinical formula and Hadlock’s ultrasound formula, and these estimates were compared against actual birth weights. The mean error of measurement was analyzed across various classes of maternal body mass index (BMI) and fetal birth weight strata.

Results: The Ultrasound Hadlock’s formula is significantly more accurate than the clinical Dare’s formula for estimating fetal birth weight, with a mean absolute error of 186.0 g versus 441.2 g (p<0.001). Ultrasound accurately estimates within 10% of actual birth weight in 83.7% of cases, compared to 37.6% for clinical estimation (p<0.001), and remains reliable across different maternal BMIs. However, the clinical method is more accurate in predicting fetal macrosomia, with a mean absolute error of 153.3 g compared to 343.3 g for ultrasound. Increasing maternal weight gain during pregnancy is also significantly associated with higher birth weight (p < 0.001, B = 0.023).

Conclusion: Ultrasound is crucial for improving the accuracy of fetal weight estimation, particularly in obese mothers. However, clinical estimation remains valuable, especially in low-risk birth centers where ultrasound may not always be accessible.

Keywords: Birth Weight, Fetal Macrosomia, Pregnancy Complications, Ultrasound Hadlock, Dare’s Formula.