The masculinization of births is a recent phenomenon resulting from prenatal sex selection. This paper reviews recent research on imbalances in the sex ratio at birth and the corresponding mechanisms, determinants and future implications. The mechanisms affecting the sex ratio at birth are reviewed, with an emphasis on factors relating to sex discrimination. The figures available provide an overall picture of skewed sex ratios since the 1980s in Asian and eastern European countries. Large differences in the male surplus are also observed across birth orders, region and social group, owing above all to the conjunction of three phenomena: a preference for male children inherited from socio-familial systems, the emergence of assisted reproductive technology enabling prenatal sex selection, and a decrease in fertility that heightens the risk of not having a son. Recent research has analysed the demographic consequences of the masculinization of births on the future composition of the population and explored how social systems may adjust to these imbalances, with political responses to prenatal discrimination still proving relatively ineffective. The article also reviews some of the main future research avenues.