The following summary seems to address this issue: https://onlinelibrary.wiley.com/doi/pdf/10.2164/jandrol.109.009381. The conclusion written by the drug company (Merck) is as follows:
In summary, a 1-mg dose of finasteride does not appear to adversely affect spermatogenesis. In addition, the level present in the ejaculate of patients taking 1 mg appears to be negligible. Thus, there does not appear to be any need to stop 1 mg of finasteride in those patients trying to conceive or in those whose partners are pregnant.