Fertility rates have declined in developed countries, but what are the immediate consequences for individuals? For many important life outcomes such as educational attainment, individuals' sibship size is a more important determinant than aggregate fertility rates. In our study, we analyze sibship size data collected from 114 surveys conducted in 26 lowfertility countries over the course of the twentieth century. We show that average sibship sizes have declined in virtually all countries. Fertility rates from the Human Fertility Database underestimate average sibship sizes, but follow similar trends for most of the time. Average sibship sizes are socially stratified, with smaller sibship sizes among higher educated parents. Yet, these disparities have declined over time, indicating convergence in most countries under study. This convergence is taking place at the upper tail of the parity distribution, i.e. for large families, but not for only-child families. Our findings reveal an important yet overlooked development of social stratification in modern societies over the past century.