Is it possible to conceive on day 20 of a 31-33 cycle and have implantation bleeding three days after intercourse?

Possible: Yes if you are sure of your cycle lengths, you may have ovulated around day 20. Typically, however, you don't see implantation bleeding until at least 6 to 12 days after conception. Check a pregnancy test around the day you expect your period. If negative, and no bleeding, try again a few days later. You need to be very careful when you have unprotected sex or strongly consider contraception.

Day 20 conception.: A 31-33 day cycle certainly is within the generally accepted range of normal cycle of 25-35 days. However, ovulation in this type of cycle does not always typically happen on day 14. Therefore it is certainly possible to conceive on day 20. After fertilization takes up to 5 days or more for implantation to take place, so a bleeding occurring 3 days after fertilization is unlikely implantation.