Today, Niger is the country with the highest estimated total fertility rate globally. Historical studies have indicated that these high fertility rates may not have been a constant in the country’s history. Instead, research on Niger and other sub-Saharan African countries has shown that fertility rates may have started increasing during the late colonial period. The goal of this paper is to test whether fertility rates indeed rose in this period and, if so, to ex plore what may have caused this increase. To do this, we draw on retrospective fertility data from the national sample census conducted in 1960–61 to estimate total fertility rates until the 1910s. To adjust for possible under registration of births, we apply a relational Gompertz model to the data for women between the ages of 15 and 49. Our estimates provide evidence for an in-crease in fertility rates that commenced around the late 1940s or, at the latest, the 1950s. Our exploration into what may have caused this fertility increase finds that declines in infertility were an unlikely factor, as primary and secondary infertility likely increased between 1940 and 1960. We argue that the most probable driver of increased fertility was the decline in birth spacing, which would have led to an increase in births per woman.