Continued high fertility in some countries is the major driver of population growth, while in other countries the decline of fertility to historically low levels propels changes in the age distribution, including the gradual ageing of the human population. Even though the past few decades have witnessed a major reduction in fertility levels worldwide, global and regional data and trends mask widespread diversity. The Population Division produces estimates and projections of fertility rates by age and of the total fertility rate at the global, regional and national levels. The Division also produces analytical studies of fertility levels and trends on a regular basis.