Accuracy of Estimated Fetal Weight below 1,000g

Abstract

The Warsof formula is 1 of 3 currently recommended by the Australian Society for Ultrasound in Medicine for the routine estimation of fetal weight. However, this formula was derived using mostly large fetuses, and its accuracy in extremely low birth-weight fetuses is not known. Using this formula, we studied 184 infants delivering within 14 days of an estimated fetal weight (EFW) < 1,000 g. The mean percent error in EFW was 11.7% underestimation, with 80% of infants underestimated and 61% of estimates within 15% error. The percent error was consistent at 9.6% throughout the first 9 days, but increased to 32% after 10 days post scan (p < 0.0001). There was a significant association between the accuracy of EFW and maternal height, but not with placental position, liquor volume or multiple pregnancy. EFW underestimated birth-weight on average by 9.6% during the first week, and this error appeared to be mainly due to the formula. The error in EFW became unacceptable after 9 days and repeating the estimate before 10 days is recommended. A different formula may be more suitable for extremely low birth-weight fetuses.

Factors Influencing the Accuracy of Fetal Weight Estimation with a Focus on Preterm Birth at the Limit of Viability: A Systematic Literature Review

“…However, the mean time interval was relatively short with 4.1 days. Surprisingly, Scott et al [8] who set the same maximum time interval of 14 days and had a mean time interval of 3.8 days found that the number of days between FWE and delivery were significantly associated with an error in FWE. While the mean percentage error at 9 days prior to delivery was 9.6%, it increased up to 32% when the FWE was done more than 9 days prior to delivery.…”

“…Only three studies have analyzed the impact of GA on the accuracy of FWE [7,8,9]. While Heer et al [7] included neonates with a GA between 22 0/7 and 42 0/7 weeks, Mills et al [9] included only neonates with a GA between 22 0/7 and 28 6/7 weeks.…”

“…Five studies further examined this aspect of time [7,8,9,10,11]. Kaaij et al [11] set the maximum time interval between FWE and delivery at 14 days and did not find an effect.…”

“…Of the 29 studies included, only five did not exclude multiple pregnancy [8,11,22,23,24]. Three examined the possible effect of multiple pregnancy on FWE [8,22,30].…”

"Estimativa ultra-sonográfica e clínica do peso fetal"

“…Nossos dados também mostraram essa tendência a superestimar o peso fetal nos grupos de baixo peso (< 1000 g), e de subestimar no grupo de fetos macrossômicos (< 4000 g), em todas as fórmulas usadas, porém com resultados melhores com o uso das fórmulas de Hadlock. Já na detecção de fetos pesando menos de 1000 gramas, Scott et al (1996) relatam que a fórmula de Warsof (DBP e CA) apresenta um erro percentual médio de 11,7%, sendo que, em nosso estudo, com o uso dos parâmetros CA e CF, obtivemos um erro de 9,46%, e 70% dos casos apresentaram uma variação de até 10% do peso no nascimento, dado esse que reforça o achado por Guidetti et al (1990) que também preferem o uso desses parâmetros fetais na avaliação de fetos com restrição de crescimento.…”