Vaginal douching may potentially wash away the viable sperm that is still in the upper vagina and this may reduce the chances of conception.
On the other hand, some patients may develop vaginal infections after intercourse if they do not douche after intercourse.
It is best to wash approximately 20 minutes after intercourse to reduce the risk of developing vaginal infections after intercourse but yet give enough time for the viable sperm to swim into the cervical canal for conception.