How years of schooling affect fertility intentions by delaying age at first marriage

China's persistently low fertility is linked to educational expansion and delayed marriage. Using data from the Chinese General Social Survey (2015, 2017, 2018, and 2021, N = 6,310) of ever-married individuals aged 20-40 years, this study examines the direct and mediating effects of the number of years of schooling and age at first marriage on fertility intentions. The results indicate the following: (1) Years of schooling indirectly reduce fertility intentions by delaying first marriage, with a mediating effect of 38.5%, confirming an 'education→delayed marriage→lower fertility intentions' pathway. (2) The effect of delayed first marriage on fertility intentions is substantially greater than the independent effect of education. (3) Gender heterogeneity exists: for men, the effect of education is fully mediated by marriage timing, whereas for women, it is only partial. (4) Subgroup analyses reveal that the mediating pathway is more pronounced among agricultural hukou holders and medium-status families, and serial mediation further shows that delayed marriage lowers fertility intentions partly through matching with more educated spouses, accounting for an additional 23.1% of the total effect. We conclude that China's low-fertility crisis is a systemic outcome of educational expansion, marriage postponement, and conflicting policy goals, with marriage and assortative mating serving as key transmission channels.