Fertility plays a crucial role in determining the quality of a country’s population. Given the rapid decline in fertility rates in recent years, Vietnam is likely to face population ageing, particularly in major urban areas such as Ho Chi Minh City and Hanoi. While Hanoi’s fertility rate remains lower than the national average, it is closely approaching it, with fluctuations following an unstable trend. In contrast, the fertility rate in Ho Chi Minh City has reached an alarming low rate - substantially below the national average and among the lowest of nearly all provinces and cities nationwide - with unpredictable shifting trends. According to demographic transition theory, economic and especially socio-cultural factors play a decisive role in driving these divergent fertility trends between the two cities.