Prenatal environmental exposures may influence sex differences during the intrauterine period, at delivery or throughout the life span of the individual. Infections, dietary factors, drug exposures or perinatal complications could differentially influence brain development in male or female offspring (eg, McCarthy et al. 32 ). Studies of rodents have shown widespread sex differences in brain structures according to the timing and dose of exposure to sex steroids, and the production of testosterone in male fetuses has been shown to induce sexual differentiation in the brain. Specifically, rodent studies have showed that even though female fetuses do not produce testosterone, they do respond to exogenous testosterone in the prenatal environment (ie, from male littermates), and remain sensitive to its masculinizing effects for a longer time than males, even postnatally. 33 For twin pregnancies, sex of the co‐twin has even been proposed to influence manifestations of neurodevelopmental disorders through effects on the intrauterine environment. For example, Eriksson and colleagues 34 investigated whether elevated levels of testosterone in utero increase the risk of developing ASD or ADHD traits, by assuming fetuses with a
