Agree with PP mostly, but would add that some REs discourage sex in the two week wait after IVF, because the cervix is often still irritated from the transfer/ultrasounds, and it can introduce bacteria and cause inflammation. (They also don't seem too keen on the "joustling" if you have OHSS, even mild cases, though I don't really think it would physically hurt the embryo.) The basic rule is not to do anything you would potentially regret if it fails, and if you get a BFN but had sex a few times after transfer, that might be the case. Though everyone is different, of course.