Purpose: This study tries to systematically understand factors that explain levels of happiness among pregnant women in the Ecological systems theory. Methods: A descriptive, cross-sectional study was conducted with 169 pregnant women in Korea. Collected data from self-report questionnaires were analyzed by hierarchical regression analysis using the SPSS statistics 23 program. Results: A total of 5 models were examined according to individual, microsystem, mesosystem, exosystem, and macrosystem in the Ecological systems theory. In the first model including individual factors, extraversion, neuroticism, and physical and psychological change constitute significant factors explaining happiness. In the second model with microsystem factors and in the third one with mesosystem factors, marital intimacy appears to be a significant factor. In the fourth model including exosystem factors, community service is a significant factor. In the final model with social atmosphere, personality (β=.15 for extraversion; β=-.30 for neuroticism), physical and psychological change (β=-.15), marital intimacy (β=.35), and community service (β= .18) turn out to be significant. These factors explain 59% of the variance of happiness in the pregnant women in Korea. Conclusion: Considering the fact that pregnant women's happiness is explained by microsystem and exosystem factors as well as individual factors, developing intervention programs that can promote influencing factors such as marital intimacy and community service is necessary to improve levels of happiness among pregnant women in Korea.