Modifiable key factors and semen quality in men undergoing preconception evaluation: a cross-sectional study.

Male preconception lifestyle factors, such as smoking, alcohol consumption, and body mass index, may affect semen quality, but their dose-dependent and nonlinear effects remain unclear. We retrospectively analyzed 3336 men attending the Department of Andrology, West China Second University Hospital, Sichuan University (Chengdu, China) between January 2019 and June 2023 and collected their demographic information, lifestyle behaviors, and semen parameters. Associations were examined using multivariable regression models with natural cubic splines, with adjustment for age, abstinence period, and season. Smoking showed a clear dose-dependent adverse effect on semen quality as shown by a decline in sperm concentrations, total sperm count, and sperm motility in men who smoked more than 20 cigarettes each day or for longer than 10 years. Former drinkers showed limited improvement in sperm motility. The body mass index showed complex nonlinear associations with semen volume and sperm morphology, and some parameters peaked in mildly overweight men, while obesity remained associated with impaired semen quality. These findings highlight the substantial role of lifestyle factors in male reproductive health during the preconception period and emphasize the importance of smoking cessation and weight management. Even modest changes in body weight may meaningfully improve semen parameters, supporting targeted lifestyle guidance in preconception care.