Objectives The aim of this study was to investigate the effects of parental consanguinity on the 2D:4D digit ratio of newborn infants, whose parents were the first cousins. Methods The study included healthy and full-term newborn infants, delivered in a hospital, at the gestational ages from 37 to 41 weeks. A total of 225 newborns and their parents were included in the study. Of these 225 newborns; 100 were inbred and 125 were outbred infants. We used a Vernier caliper to measure the length of the second and fourth digits of the newborns (accuracy: 0.01 mm). Results Controlling for education and number of pregnancies, male newborns had lower digit ratios than female newborns, for both left and right hands. There were, moreover, differences between inbred and outbred samples. We determined that inbreeding was related to a reduction in the digit ratios regardless of the side of the hand and the sex of the infant, showing a consistent tendency to appear more masculine. Interaction between marriage type*sex was highly significant. Conclusions Parental consanguinity appears to cause fetal masculinization of digit ratios by increasing fetal stress among Turkish newborns.