A mother giving birth in December can claim child-related tax benefits when she files her tax return a few months later. Mothers of January-born children must wait more than a year before receiving child-related tax benefits. Thus, families with December births have higher after-tax income in the first year of a child’s life than otherwise similar families with January births. This paper estimates the corresponding income effect on maternal labor supply. December mothers have a lower probability of working, particularly in the third month after a child’s birth, and we find some evidence that child-related tax benefits reduce maternal earnings.