What are the chances of getting pregnant only having unprotected sex one time midcycle?

Depends. Your age is a major determinant; the chance may be as high as 25% in your teens and twenties, but decreases to <5% after 40.
Less than 25% Average fertile woman having unprotected intercourse has approximately 25% chance of conceiving in one cycle. If you have only one intercourse in the midcycle, your chance will be lower than this.

Related Questions

Are your chances of getting pregnant less likely if you have sex during a missed period? I had unprotected sex 5 times during one and am still late.

Pregnancy test. If you're asking whether or not it's possible to get pregnant during aperiod , that'd be highly unlikely because you're shedding the soil for which a seed could be implanted. The fact that you're missing periods though and having unprotected sex means that you should get a pregnancy test and if it's negative see a doctor to figure out why you're missing them. Read more...