Context: Recent studies showed that male reproductive health problems, such as cryptorchidism, hypospadias, testicular cancer, and low sperm quality, are more prevalent in Denmark than in Finland.
Objectives: We hypothesized that, if fetal testicular dysgenesis contributed to these observations, differences in gonadal development and the hypothalamus-pituitary-testis axis would already be detectable perinatally. Thus, we investigated healthy newborn boys in both countries.
Design: This was a prospective, longitudinal population-based study.
Setting: Two primary obstetric centers were included at the University Hospitals of Copenhagen, Denmark, and Turku, Finland.
Participants: The participants of the study included 633 Danish and 1044 Finnish boys, born at term with appropriate weight for gestational age.
Interventions: Ultrasound determination of testis size at 0, 3, and 18 months and blood sampling (n = 727) at 3 months were analyzed.
Main outcome measures: Testicular volume and reproductive hormones were measured.
Results: Testis volume was significantly higher at all ages in Finnish than in Danish boys (medians, 98 vs. 95, 185 vs. 119, and 188 vs. 136 mm(3), respectively; P < 0.00001). Testis growth from birth to 3 months was larger in Finnish than in Danish boys (mean, 75 vs. 26 mm(3); P < 0.0001). Serum hormone levels were higher in Finnish than Danish boys for inhibin B (median, 456 vs. 385 pg/ml; P < 0.0001), FSH (1.33 vs. 1.21 IU/liter; P < 0.036), and SHBG (143 vs. 136 nmol/liter; P < 0.022). Inhibin B was significantly positively correlated to testicular volume (r = 0.25; P < 0.006).
Conclusions: The larger testes and higher inhibin B levels most likely represent a bigger volume of seminiferous tubules in Finnish compared with Danish boys. Although this phenomenon may be attributable to a genetic difference between the two countries, it may also reflect environmental factors influencing testicular development.