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Nope, no such device exists, though I'm sure there would be a pretty good market for an invention along these lines.  If someone is worried about becoming pregnant after having unprotected sex then she might want to consider taking emergency contraception pills (often called the morning after pill).  EC pills are pretty much like regular birth control pills, only you take them after you have sex and they aren’t recommended as a regular form of birth control (because you can have irritating side effects like irregular periods and they just aren’t as reliable as most other forms of contraception).  Though people generally get a prescription for a specific brand of EC, if this isn’t available to you, you can check out this chart that shows which regular birth control pills can be taken as EC and how.   It is really important to take EC as soon as you can after sex because they are most effective when taken within 72 hours after unprotected sex (though it is still worthwhile to take them up to 120 hours after sex). You can find EC by calling 1-888-NOT-2-LATE.  Hospitals and your health care provider should also be able to hook you up with EC.  Though EC can reduce the risk of pregnancy by 75 to 89 percent, it will do nothing to prevent sexually transmitted infection.