Comparison of accuracy of Johnson and Risantos formula for fetal weight estimation in term parturients in Mulago Hospital

Abstract

Introduction: Precise and accurate estimation of birth weight (BW) is one of the most important aspects at the start of labor. Estimation of fetal weight using measurements of fundal height (FH) and abdominal girth (AG) is beneficial especially in developing countries where ultrasound facilities are either unavailable or unaffordable by patients in majority of rural facilities. Estimation of BW by palpation gives subjective results with accuracy depending on experience, which may be lacking in many obstetric care personnel in developing countries. In Mulago National Referral Hospital 24.2% of the emergency cesarean sections are due to obstructed labor, 3.6% big baby and 5.8% due to cephalopelvic disproportion. There was no recommended method for clinical estimation of fetal weight at the time of the study. This study aimed at comparing the accuracy of Johnson’s and Risanto’s methods of fetal weight estimation in Mulago National Referral Hospital. Methods: This was a hospital based cross sectional study involving 334 parturients admitted in labour. After obtaining informed consent, a pre-tested semi-structured questionnaire was administered and quantitative data was collected. Fundal height was measured by two research assistants during a no contraction period using flexible non-elastic tape (sewing tape) from the upper margin of symphysis pubis to the top of the uterine fundus in the midline. While ensuring privacy and comfort a pelvic examination was performed. After birth of the baby, the weight of the new born was taken within one hour. Data was double entered and cleaned data using Epidata version 3.5 and then the data exported to SPSS version 20.0 package for analysis. Percentage error was calculated as the absolute weight difference divided by the actual birth weight, multiplied by 100 and errors within 10% was considered as accurate. Comparison of accuracy of Johnson and Risanto’s method was done using a paired t test. Approval for this study was sought from the Directorate of Obstetrics and Gynecology and School of Medicine Research and Ethics Committee. Results: The mean age was 26.3 ± 5.7 years. One hundred thirty-four (39.9%) had normal body mass index, ninety-seven (29.0%) were overweight while forty-one (12.2%) were obese. The mean maternal height was 167.4 ± 10.8 cm; the mean maternal weight was 69.8 ± 11.9 kg. Two hundred and fifteen (64.0%) mothers were multigravida. The Mean Symphysio-fundal height was 36.69 ± 1.06 cm. The mean actual birth weight was 3231.8 ± 378.1 gms while using Johnson’s and Risanto’s formula the mean weights were 3814.0 ± 182.0 gms, 3707.3 ± 134.5gms respectively. The overall accuracy of Johnson’s formula was 25.9% while that of Risanto’s was 31.5%. However, for actual birth weights between 3500.0gm – 4000.0gm an accuracy of 85.9% using Johnson’s formula and 97.8% using Risanto’s formula was observed. Johnson’s formula revealed weight differences ranges of 1.0gm to 1376.0 gms with the mean absolute weight difference of 582.9gms CI 546.9 – 619.0, p value 0.000 while Risanto’s formula revealed a weight difference range of 3.0 gms to 1155.0 gms. The mean weight difference between Johnson and Risanto’s formula was 107.4gms; CI 93.3 – 121.5, p value 0.000. Discussion/Conclusion: Both Johnson’s and Risanto’s formulae are not accurate at estimating fetal weights in Uganda. Risanto’s formula is significantly more accurate than Johnson’s formula in estimating fetal weight in Mulago Hospital, Uganda. However, both methods are accurate at estimating birth weights above 3500.0 gms. Therefore both methods should not be used for routine fetal weight estimation in Uganda. Nevertheless, more studies need to be undertaken to determine which clinical methods are to be useful in estimating fetal weight in general in Uganda.