Some research suggest that increased male age is associated with a decline in semen volume, sperm motility, and sperm morphology. In studies that controlled for female age, comparisons between men under 30 and men over 50 found relative decreases in pregnancy rates between 23% and 38%. It is suggested that sperm count declines with age, with men aged 50–80 years producing sperm at an average rate of 75% compared with men aged 20–50 years and that larger differences are seen in how many of the seminiferous tubules in the testes contain mature sperm: