Abstract: This study proposes a hypothesis of social differentiation effects of extended maternity leave to reinterpret the “fertility-unemployment paradox” caused by extended maternity leave among migrant women. In 2016, China introduced revisions of extended maternity leave to the provincial Population and Family Planning Regulations. This paper examines the aftereffect of the 2016 and 2018 CMDS data and tests the hypothesis empirically by using the propensity score matching and instrumental variable method. The study finds that extending maternity leave can effectively improve migrant women's willingness to have a second child, but at the same time it significantly increases the risk of unemployment for them. The probability of unemployment increases by 11.76% for every month of maternity leave extension. This conclusion remains valid after a series of robustness tests. In addition， there is a threshold value for the impact of extended maternity leave on the second child fertility intention. When the length of maternity leave exceeds 180 days， further extension can no longer increase migrant women’s desire to have a second child. Further examination finds that the “fertility-unemployment paradox” of maternity leave extension is only a symptom. The maternity leave extension policy with its original intention of supporting families and protecting women creates a social differentiation effect among migrant women， that is， the extension of maternity leave only increases the second child fertility intention of migrant women with high socioeconomic status in terms of education， income and formal employment， but it puts migrant women of low socioeconomic status such as rural migrant workers with low education in a much greater risk of unemployment. As a family policy intended to protect vulnerable groups， the maternity leave extension policy has instead created new inequalities in its implementation， causing more difficulties for migrant women of low socioeconomic status. This study not only contributes to the research on low fertility， but also provides theoretical reference for improving public policies and guiding the development of modern families.

Key words: the extension of maternity leave, fertility-unemployment paradox, social differentiation