Background: Accurate gestational age (GA) estimation is crucial in maternal and perinatal health research. However, the fixed-duration last menstrual period (LMP) estimation method has not been validated in Korea. Objectives: We utilized the Korea’s claims data, linked with the Korea Immunization Registry Information System (KIRIS), which provides data on GA at the time of vaccination. This study aimed to validate a deterministic GA estimation algorithm based on fixed-duration LMP estimation, by comparing the estimated LMP with those calculated from KIRIS (considered the gold standard). Methods: Based on the GA at vaccination recorded in KIRIS, the gold standard LMP was calculated by subtracting it from the vaccination date, with an additional 3-day adjustment to better approximate the pregnancy start. We then estimated LMP using our GA estimation algorithm across various pregnancy outcomes by subtracting a fixed duration from the pregnancy end date: 39 weeks for live births, 10 weeks for spontaneous and induced abortions, 28 weeks for stillbirths, and 8 weeks for ectopic pregnancies. Finally, we compared the estimated LMP with the gold standard LMP by evaluating the accuracy for each pregnancy outcome. Accuracy was assessed by the proportions of estimated LMPs falling within 1-4 weeks of the gold standard among pregnancy episodes stratified by each pregnancy outcome. Results: Among 357,229 pregnancy episodes, accuracies at the two-week threshold were as follows: live births, 92.0% (319,460/347,089; 95% CI: 92.0-92.1); induced abortions, 55.6% (10/18; 33.7-75.4); stillbirths, 13.4% (19/202; 6.1-14.2); spontaneous abortions 56.2% (1,658/2,952; 54.4-58.0); and ectopic pregnancies, 21.4% (12/56; 12.7-33.8). The four-week threshold did not show a significant improvement in accuracy. Conclusion: The fixed-duration LMP estimation method is reliable for live births within the Korean healthcare data. However, there is a need for an improved model for non-live births, particularly stillbirths, to reduce their GA misclassification.