Comparative Study of Various Methods of Fetal Weight Estimation at Term Pregnancy

INTRODUCTION: The present study was aimed at estimation of fetal weight in utero by some of the different formulas that have evolved over time. Accurate estimation of fetal weight is of paramount importance in the management of labour and delivery. During the last decade, estimated fetal weight has been incorporated into the standard routine antepartum evaluation of high-risk pregnancies and deliveries. For instance, management of diabetic pregnancy, vaginal birth after a previous caesarean section, and intrapartum management of fetuses presenting by the breech will be greatly influenced by estimated fetal weight Obstetric ultrasound has in fact revolutionized the knowledge of fetal medicine in the present day and with its diagnostic modality, one logically should be able to predict fetal weight with a great degree of precision. AIMS AND OBJECTIVES: The objective of this study is to evaluate the accuracy of antenatal assessment of fetal weight in term pregnancies by using (a) Two clinical formulas; 1. Abdominal girth (cms) x symphysio - fundal Height (cms), 2. Johnson’s formula. (b) Ultrasonographically estimating fetal weight using Hadlock formula by measuring HC, AC, and femur length. Comparison of the different methods with the actual birth weight of these babies after delivery. MATERIALS AND METHODS: This longitudinal cross sectional study was carried out at the Institute of Obstetrics and Gynaecology, Egmore, Chennai. This study was approved by the Institution’s ethical committee and was conducted between August 2009 to September 2010. Patient Population: 1000 cases of randomly selected term pregnancies were included. Inclusion Criteria: All pregnant women admitted at full term for planned delivery either by elective caesarean section or by induction of labour. Mothers with live singleton fetus who had their gestational age confirmed by dates and ultrasound done before 22 weeks. All measurements were taken within one week of delivery. If undelivered beyond this time interval the measurements were repeated. Exclusion Criteria: • Multiple gestations, • Patient with polyhydraminos or oligohydramnios, • Abnormal lie, • Preterm labour, • Fetal malformations, • Antepartum hemorrhage, • Eclampsia, • Obese patients (>90 kg), • Uterine / ovarian mass complicating pregnancy. SUMMARY: This study was undertaken at the Institute of obstetrics and gynaecology, Egmore, Chennai to compare the various methods of fetal weight estimation at term pregnancy in 1000 patients. The cases were randomly selected and detailed obstetrical history was taken. The gestational age of all the patients was known and all the cases delivered within one week of measurement. Fetal weight was estimated by using different formula and was compared to the actual weight of the baby taken immediately after birth and a comparative analysis was done. 1. Of the 1000 cases, 454 were primigravida and 546 were multigravida. 2. Most of the women were in the average reproductive age group of 20-30 years. 3. Most of the patients had normal vaginal delivery (53.9%) and 39.7% delivered by lower segment caesarean section. 4. The sex distribution of the babies in the study population showed that more male babies were born. 5. Majority of the babies at birth weighed between 2501- 3000 gms. 6. In the study population, more primigravida delivered babies with very low birth weight and more multigravida delivered babies of birth weight > 3500 gms. 7. Johnson’s and ultrasound-Hadlock’s formula had a marked tendency to overestimate the fetal weight. 8. Error was within 350 gms in 84.7%, 70.8% and 84% of cases by Dare’s, Johnson’s and ultrasound-Hadlock’s formula. 9. The mean simple error and the mean absolute error was least by Dare’s formula followed by ultrasound-Hadlock’s and Johnson’s formula. 10. The mean absolute percentage error was least by ultrasound – Hadlock’s formula followed by Dare’s formula and Johnson’s formula. 11. The coefficient correlation calculated for different methods showed that ultrasound seems to be correlating well with actual birth weight than Dare’s and Johnson’s formula. 12. The estimates within 10% of actual birth weight was 67.3%, 62.7% and 59.9% with Dare’s, Johnson’s and ultrasound-Hadlock’s formula which was not stasticially significant. 13. Antenatal assessment of birth weight by ultrasound seems to be better for estimating low-birth weight babies and for large for gestational age babies. CONCLUSION: Clinical estimation of birth weight may be as accurate as routine ultrasonographic estimation, except in low-birth-weight babies. Therefore, when the clinical method suggests weight smaller than 2,500 g, subsequent sonographic estimation is recommended to yield a better prediction and to further evaluate the fetal well-being. From our study, it can be concluded that antenatal fetal weight can be estimated with considerable accuracy by abdominal girth X symphysio-fundal height and ultrasound – Hadlock’s formula. Abdominal girth x symphysio-fundal height is simple, inexpensive and of immense value in developing country like ours, hence it can be used anywhere even by domiciliary midwives to predict fetal weight. Accuracy of Johnson’s formula was less than Abdominal girth x symphysio-fundal height and ultrasound – Hadlock’s formula.