The Chinese fertility rate has fallen to an extremely low level, with the decline in first-birth fertility constituting the primary driver of overall fertility reduction. It is increasingly essential to explore factors influencing the first-birth intentions of childless couples. Utilizing data from a specialized survey on never married groups, childless and single-child couples and employing a bivariate Probit model, this study examines how intergenerational coresidence influences the first-birth intentions of married childless couples. The findings reveal that coresidence with paternal parents significantly enhances husbands' and wives' first-birth intentions among both urban and rural couples, while coresidence with the maternal parents exerts a significant positive effect exclusively on urban couples. When coresidence stems from housing constraints, such arrangements partially counteract the positive impact of intergenerational coresidence on urban couples' fertility intentions. For rural couples living with paternal parents, couples' fertility intentions increase with husbands' share in marriage costs. These findings demonstrate the complex influence of intergenerational coresidence on couples' fertility intentions within the Chinese context, deepening our understanding of the interplay between family structure and reproductive decision-making. In light of persistent low fertility, fertility support policies should promote intergenerational support norms and provide targeted housing subsidies to alleviate structural constraints on childbearing.