We show that laws mandating use of child car safety seats significantly reduce birth rates, as many cars cannot fit three child seats in the back seat. Women with two children younger than their state’s age mandate have a lower annual birth probability of .73 percentage points. This effect is limited to births of third children, households with access to a car, and households with a male present, where both front seats are likely to be occupied. We estimate that these laws prevented fatalities of 57 children in car crashes in 2017 but reduced total births by 8,000 that year and have decreased the total by 145,000 since 1980.