Accuracy of Johnson's Formula for Estimating Fetal Weight Compared to Actual Birth Weight: A Study of 600 Cases in Rangpur Medical College

Introduction: The estimation of fetal weight during pregnancy has a significant impact on the survival of newborns and some immediate and late sequelae of the life of newborns. Estimation of fetal weight also plays a paramount role in determining time, mode, and place of delivery. There are many methods to estimate fetal weight like the palpation method, fundal height measurement, and radio frequency volume reduction. Johnson's method is one of the clinical methods that require no expense and is easier to estimate the precise birth weight and fetal weight. This study aimed to analyze the accuracy of the Johnsons formula for estimating fetal weight compared to actual birth weight. Methods: This cross-sectional prospective study took place in the Gynecology and Obstetrics Department of Rangpur Medical College Hospital, Rangpur, between May 2015 to October 2015, six (06) months after approval. The sample size was 600 as per inclusion criteria. Data were collected through face-to-face interviews with the women by using a pre- design questionnaire after proper counseling and informed written consent. The purposive sampling method was used in the study. Data processing was consisting of registration schedule, editing, computerization, preparation of dummy table, analyzing & matching of data. Statistical analysis was carried out by using the Statistical Package for Social Sciences version 16.0 for Windows. The mean values were calculated for continuous variables. The quantitative observations were indicated by frequencies and percentages. Paired t-test was used for continuous variables. Pearson's correlation coefficients were used to test the relationship between the groups. P values <0.05 was considered statistically significant. Result: The mean age was found 24.5±5.1 years with a range from 18 to 38 years. The mean symphysio fundal height was found 32.4±2.0 cm with a range from 28 to 36 cm. The mean estimated fetal weight was found 3205.2±287.9 grams with a range from 2635 to 3875 grams. The mean actual birth weight after delivery was found 3019.0±359.0 grams with a range from 2200 to 3800 grams. The difference was statistically significant (p<0.05) between the two groups. A positive significant correlation (r=0.929; p=0.001) was found between actual birth weight and estimated fetal weight by Johnson's formula. A positive significant correlation (r=0.517; p=0.001) was found between actual birth weight and symphysio fundal height. A positive significant correlation (r=0.129; p=0.002) was also found between the actual birth weight neonate and the BMI of the mother. Conclusion: Antenatal and intranatal fetal weight can be estimated with reasonable accuracy, clinically using Johnson's formula. Ultrasound is not available in remote areas where Johnson's formula is easy and simple to calculate and can be included in the training program of medical and paramedical staff and birth attendants.