How long after sex does conception happen?
I have and IUD is it possible to get pregnant?

Any time you have sex, pregnancy is possible. There is no form of birth control that's 100% effective. So yes, you could be pregnant.

As for your other question, this video may help;

http://www.google.ca/url?sa=t&rct=j&...PBKpD8TBGWDSZA

Anytime you have penile insertion there is a risk of pregnancy. How long it can take for actual fertilization doesn't really matter. The only thing that matters is that you can't tell if conception occurred until a couple of weeks after you miss your period.