Japan has faced a decline in fertility since the coronavirus disease 2019 (COVID-19) pandemic. This study aimed to investigate the rate of pregnancy postponement and its contributing factors, with a particular focus on economic- and COVID-19 infection-related indicators. This study used data from 768 observations of married women aged 18 to 50 years with pregnancy intentions. The data were obtained from two rounds of a large web-based survey conducted by the Japan COVID-19 and Society Internet Survey (JACSIS) in 2020 and 2021. A generalised estimating equation (GEE) model was employed, as well as Poisson regression models for sub-sample analysis divided by year to estimate the year differential magnitude of the contributing factors’ impacts. Approximately 20% of married women with childbearing intentions postponed their childbearing. The analyses revealed that declining income and anxiety about future household finances were significantly related to delayed childbearing, while fear of COVID-19 and infection rate were not. Additionally, the adverse effects of unfavourable economic conditions were stronger in 2021. Notably, age did not influence the decision of pregnancy postponement. Older women postponed pregnancy just as much as younger women. In conclusion, this study confirmed that the COVID-19 pandemic, particularly its related adverse economic conditions, contributed to Japan’s current baby bust. Considering that advanced maternal age is already common in Japan, this decreased fertility may result in the long-term negative consequence of further population decline.