A new study says that being chubby may reduce a man's chances of becoming a father, because fat zaps his sex hormones.
To reach the conclusion, the researchers reviewed 87 healthy men aged 19 to 48 - around two-thirds of whom were already dads.
The research team calculated the amount of fat they had - their Body Mass Index (BMI) - from their height and weight.
From the analysis, the team found that men with a BMI of 32 were least likely to father a child - while lower levels of sex hormone testosterone were found in every overweight man.
Two other key fertility hormones - luteinizing and follicle-stimulating - were also depleted.
The finding implies that the testicles, where sperm is produced, are not working properly.
Head researcher Dr Eric Pauli, of Pennsylvania State University College of Medicine, said couples being assessed for unexplained infertility should take the weight of both partners into account.