This is a artical from Dave palumbo in md's mag:
''Testosterone stimulates the sertoli cells in the testicles to produce sperm. In order for testosterone to accomplish this task, it needs to present at relatively high concentrations (i.e., it needs to ''bathe'' the sertoli cells in testosterone). Whenever you inject an exogenous source of testosterone, the leydig cells in your testicals stop producing endogenous testosterone. Whenthis occurs, this locally produced source of testosterone is no longer available to stimulate the sertoli. Say for instance, you inject 250 milligrams of testosterone enanthate per week. By the time this 250 milligrams gets into the bloodstream, it's been watered so significantly that the relative amount passing through the testicals winds up being negligible. However, as levels of exogenous testosterone injections rise to very high levels (over 1,000 milligrams per week), the relative amount passing through the testicals now increase to ample levels to cause spermatogenesis to occure once againg. When this happens, lo and behold, your fertile andbefore you know it you're girlfriend's pregnant. This could explain pregnancies while on."