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&q=how%20pregnancy%20happens%20video&source=web&cd=3&cad=rja&sqi=2&ved=0CEMQtwIwAg&url=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3Do3u OILAW6Lw&ei=20mxUIm2EZHkiwL1-IDYCg&usg=AFQjCNES0SNZY2dX8ujVPBKpD8TBGWDSZA

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.