Prenatal ultrasound can accurately predict fetal weight, so as to timely detect giant infants, reduce unnecessary trial delivery, and avoid maternal and infant injuries such as cervical laceration, shoulder dystocia, bone and brachial plexus injury[19–21]. It can reduce the increase of selective cesarean section rate caused by incorrect EFW and insufficient confidence of pregnant women in trial delivery. It can also be found that very low weight infants, fully estimate the tolerance of the fetus to hypoxia during contractions, timely choice of cesarean section termination of pregnancy, to avoid adverse perinatal outcomes[23–24].
Both Kiserud et al. and Gardosi et al. showed that there were normal physiological changes between different countries and nations, and a single universal standard was not appropriate for everyone. Therefore, the purpose of this study was to create and verify the optimal regression equation for predicting fetal weight, according to the population characteristics of Chongqing.
In previous studies[4–6], they all ignored the difference between ultrasonic measurement time and birth time, and directly used the data measured during the last ultrasonic examination to correspond to birth weight, thereby calculating the estimated fetal weight, which would result in a large estimate. A weight compensation mechanism was established to compensate for the difference between the weight measured by ultrasound and the birth weight. We found the mean absolute error of the Chongqing equation established in this study was 179.06 g, 171,63 g, 175.88 g, 192.20 g and 174.54 g respectively from 38 to 42 weeks of gestation, which was the smallest error of the four equations in each gestational period. The standard deviation of the Chongqing equation was 139.14 g, 136.76 g, 146 g, 136.57 g and 126.64 g respectively from 38 to 42 weeks of gestation, which was the smallest standard deviation of the four equations in each gestational period. It could be seen from Fig. 1 that except for 37 weeks, the estimation error of Chongqing equation was the smallest for other gestational weeks. This might be because there were only 63 pregnant women collected at 37 weeks, with a small sample size. Further discussion is needed.
The optimal regression equation for EFW in this region was established through the weight compensation mechanism. As Skupski research showed, the significant difference of the formula couldn’t be calculated. The accuracy of the new equation and other representative equations was prospectively verified by comparing the estimation error. We analyzed not only the estimated error of each gestational week, but also the overall estimated error. The estimation error was the error of estimation weight and compensation weight, and the error of estimation weight and birth weight. The estimated weight per gestational week estimated by the Chongqing equation at 37 to 42 weeks of gestation was the closest of the four equations to the actual average birth weight. In the sets established by 1925 data, the mean absolute error and standard deviation of the estimation error of the equation established in this study were 178.9 g and 140.3 g respectively. In the 300 validation sets, the mean absolute error and Standard deviation of Chongqing equation were 173.08 g and 128.59 g respectively. The equation established in this study has the smallest mean absolute error and the smallest standard deviation. Therefore, compared with other equations, the fetal weight estimation equation established in this study was more applicable to Chongqing fetuses, and it could be used to obtain more accurate fetal weight estimation of Chongqing fetuses.
This study has several unique advantages over previous studies. Firstly, we excluded ethnic and geographic factors and used multiple reverse elimination regression technique and the ten-fold cross verification method to establish the fetal weight prediction model. The method of deriving the equation can be extended to China and other parts of the world to establish a suitable reference equation for local fetal weight estimation. Secondly, we collected a large amount of data, specifically 1925 cases. They were carefully screened to ensure good health, excluding ethnic, geographic and other factors. Not only fetal structural abnormalities that may lead to adverse pregnancy outcomes were excluded, but patients with pregnancy complications were also excluded, all of which may affect the growth rate and development of the fetus, and affect the derivation and establishment of this formula. For example, the inclusion of gestational diabetes may increase the incidence of premature fetuses and macrosomia, leading to a decrease in the popularity of the equation. Finally, from the analysis of the comparative results, the equation established in this study had the least error and was most suitable for the prediction of Chongqing fetal weight. Compared with the representative equation, it was proved that the application of this equation had high clinical guidance and reference value for the monitoring and management of fetal weight.
The limitation of this study is that the sonographers can see the data automatically display on the screen after measuring it, which may lead to the deviation of the expected value. Although we recruited well-trained ultrasound operators specifically instructed by the research procedure using internationally accepted techniques, different ultrasound doctors had slight operational difference that may lead to non-systematic errors. This study only collected data on pregnant women and fetuses from a hospital in Chongqing. The amount of data collected per gestational week was uneven.