JOHNSON’S formula for predicting birth weight in pregnant mothers at Jimma university teaching hospital, south western Ethiopia

Abstract

Background: Accurate assessment of fetal weight is mandatory for obstetric management of labouring mothers. Among the methods of fetal weight estimation, Symphysis fundal height measurement is an easier method of fetal weight estimation and has been shown to be as good as ultrasound estimation at term, giving estimates that are correct to within 10% of the birth weight in 60% to 70% of cases. The values of SFH are converted by using Johnson’s formula to estimate fetal weight, where birth weight in gm = (SFH in cm–12)×155, with further adjustments based on engagement of the fetal head and maternal obesity. Objective: the objective of this study is to validate Johnson’s formula for predicting birth weight in pregnant mothers at JUSH, south western Ethiopia, 2014. Methods: A prospective cross sectional study was conducted in Jimma University Hospital. 334 mothers admitted to the labor and maternity ward who fulfill all the inclusion criteria were selected till the desired sample size is achieved. Data on socio-demographic and obstetric characteristics were collected using a pre-tested structured questionnaire. Data was analyzed using Statistical Package for Social Sciences for windows version 20. Results: The accuracy of Johnson’s formula was thirty eight percent. The mean percentage error in fetal weight estimation using the formula is 17.5 percent. Gestational age , SFH and birth weight have significant effect on accuracy of Johnson’s formula. Simple linear regression between SFH and birth weight gave a correlation coefficient (r) of 0.623. Gestational age , Symphysis fundal height , BMI, sex of the neonate and membrane status are significantly associated with birth weight. Substituting the mean SFH ( 35.58cm) to the derived formula Weight in Gm= 2600 + 115(SFH(cm)- 30 ) will give us an estimated birth weight of 3175 gms while the mean estimated fetal weight by Johnson’s formula is 3565. The mean birth weight was 3244 gms. CONCLUSION Johnson’s formula was found to be inaccurate in this study particularly in LBW babies. But in macrosomic babies it is considered accurate although the small sample size in this group precludes a firm conclusion. The formula inaccurately predicts birth weight in preterms and at terms between gestational age thirty seven and fourty one weeks and six days. But is accurate at gestational age greater than 42 completed weeks despite small sample size in this group. Birth weight is strongly correlated with SFH which can be transformed by a simple clinical formula