Well, your chances any given month, when you have unprotected sex close to the time of ovulation is approximately 20%. Assuming sperm lives 72 hours, your chances could be as high as 20%.
BTW, although it is obviously possible to conceive on the day of ovulation, it is just as likely to conceive from unprotected sex that you had the day before ovulation (and possibly even longer than that).
Good luck!
Rachel Inbar
www.FertilityStories.com