Can having sex induce labor?

Possible: There is a theory that since there are prostaglandins in semen, and since certain prostaglandins can induce labor, that having sex can either chemically, or mechanically (banging on the door - so to speak) induce labor. There needs to be more than that. You must also have an issue that makes you more prone to going into labor like a preterm labor risk or a past due pregnancy.

Yes: Yes, if late in pregnancy -- but it's anatomically difficult.