Since maternal pre-pregnancy BMI may determine the adult sons’ own fat mass and BMI (Tequeanes et al., 2009), and since overweight and obesity in adult males may be associated with low fecundity, potentially owing to associations with lower levels of testosterone and SHBG and higher levels of oestradiol (Salas-Huetos et al., 2021), we examined the potential mediating role of sons’ fat mass and BMI. This evidence concerns the gene SHBG and obesity due to melanocortin 4 receptor deficiency.