CONTEXT: It remains unknown how the postnatal activation of the hypothalamic-pituitary-gonadal axis in infancy, also known as 'minipuberty', relates to adult testis function.

OBJECTIVE: To investigate how markers of reproductive function in three-month-old boys correlate with adult reproductive health parameters.

DESIGN: Population-based birth cohort study (the Copenhagen Mother-Child cohort).

SETTING: Copenhagen University Hospital, Denmark.

PARTICIPANTS: 259 boys examined once around three months of age and again at 18-20 years.

MAIN OUTCOME MEASURES: Reproductive hormones, penile length, testis volume, and semen quality. Minipubertal markers of testis function (by tertiles, T1-T3) were explored as predictors of adult semen quality using linear regression models. Associations between reproductive outcomes in infancy and young adulthood were estimated by intra-class correlation coefficients (ICCs), describing how well measurements in infancy correlate with those in adulthood.

RESULTS: Serum testosterone concentration in infancy was positively associated with adult total sperm count. Median (IQR) total sperm count was 84 (54-138) million spermatozoa for boys in T1, 141 (81-286) million spermatozoa in T2, and 193 (56-287) million spermatozoa in T3. We found the highest ICC for FSH (0.41, 95% CI 0.26-0.57), while ICCs for inhibin B, SHBG, penile length, and testis volume ranged between 0.24 and 0.27. ICCs for LH, total and free testosterone were lower and statistically non-significant.

CONCLUSIONS: Serum testosterone in infancy was a predictor of adult total sperm count. Other reproductive hormones and genital measures showed good correlation between infancy and adulthood, suggesting that an individual's reproductive setpoint starts shortly after birth in boys and persists until adulthood.