Narrative Review of National Health Insurance Claims Data for Gestational Age Calculation and Vaccine Safety Evaluation in Pregnancy

Pregnant women comprise a high-risk population, and therefore timely evaluation of vaccine safety and effectiveness is essential for them.National health insurance claims data enable large-scale observational studies but often lack essential obstetric details.This review compares methods for assessing safety of pregnancy vaccines using claims data, with a focus on gestational age estimation.We propose a standardized approach using the Korea Disease Control and Prevention Agency-COVID-19-National Health Insurance Service cohort (K-COV-N cohort) motherinfant dataset, and discuss strategies to enhance the reliability and applicability of real-world vaccine safety assessments.We conducted a narrative review of the methods used to assess vaccine safety during pregnancy using Korean NHIS claims data.PubMed was searched with "vaccine", "safety", "pregnan*", "national, " and "insurance" to identify studies on maternal vaccine exposure and its impact on obstetric or neonatal outcomes.We focused on pregnancy outcome definitions, event dating, pregnancy onset estimations, and gestational age calculations.The methods used in the K-COV-N cohort were also reviewed, and preterm birth and congenital anomaly rates were compared with national statistics.Among the 24 studies found, four met our inclusion criteria (two from Korea, one from Taiwan, and one from France).Methods for estimating pregnancy start date varied.The Korean studies derived date of last menstrual period from first-trimester ultrasound data, whereas the Taiwanese and French studies used gestational age and physician records.In the K-COV-N cohort, the calculated preterm birth rate was 6.6%, and the congenital anomaly rate ranged from 6.8% to 7.2%, consistent with national statistics.Gestational age estimation using first-trimester ultrasonography aligns with national statistics and enables rapid vaccine safety assessment; however, further validation is recommended.