Ramya Vijayram , Nikhita Damaraju , Ashley Xavier , Koundinya Desiraju , Ramachandran Thiruvengadam , GARBH-ini , Raghunathan Rengaswamy , Himanshu Sinha , Shinjini Bhatnagar , medRxiv (2020) .
Background: Different methods and formulae have been developed for different populations for estimation of GA in the first trimester of pregnancy. In this study, we develop an Indian population-specific GA dating formula and compare its performance with the previously published formulae. Finally, we evaluate the implications of the choice of dating method on preterm birth (PTB) rate. The data for this study was from the GARBH-Ini cohort, an ongoing (2015-2019) longitudinal pregnancy cohort of North Indian women to study PTB. Methods: Comparisons between USG (Hadlock) and LMP-based dating methods were made by studying the distribution of their differences by BA analysis. Population-specific dating formula for the first trimester of pregnancy (Garbhini-1 formula) was developed by constructing a regression model for GA as a non-linear function of CRL, which was then compared with published formulae by BA analysis. The PTB rate was estimated by using each of these methods and expressed as prevalence and 95% CI Results: LMP-based method overestimates GA by three days as compared to USG (Hadlock) method with limits of agreement between -4.39 and 3.51 weeks (95% CI). CRL is the most critical parameter in estimating GA in the first trimester. No other clinical or socioeconomic parameter enumerated in the GARBH-Ini cohort study contributes to GA estimation. The GA estimated by all the formulae compared showed an agreement within a week with the uppermost and the lowermost limits of agreement (LOA) being -0.46,0.96 weeks. The estimated PTB rate across all the formulae ranged between 12.12 and 16.85% with Garbhini-1 formula estimating the least rate. Conclusions: Our study reinforces the fact that CRL-based USG method is best for estimation of GA in the first trimester and addition of clinical and demographic features does not improve its accuracy. Garbhini-1 formula developed from our population data performs at par with the existing formulae but estimates the lowest PTB rate with better precision than other formulae. The applicability of Garbhini-1 formulae for the rest of the Indian population needs to be validated in subsequent studies.