What would cause a thigh to be numb?

Meralgia parestheica. The lateral femoral cutaneous nerve can get pinched causing a numb area on the front of or outside part of the thigh. Get an exam by a doctor though because your symptom may also be due to other more serious causes.
Pinched nerve. This could be due to "pinched" nerves. Where is the big question. It could be at the level of the spine, within a muscle or it could be a small superficial peripheral muscle. If it really is just a patch of numbness on your thigh it could be "meralgia paresthetica" a benign condition. But to be sure have it examined by your doctor.

Related Questions

I have broken my pelvis in 3 places and my saccrum. What would cause my left thigh to be numb to my knee?

Femoral nerve. Broken pelvis can be a cause of femoral nerve damage. Also if you consider the mechanism of trauma causing pelvic fracture, i wonder if there is associated l2-3-4 spine injury. You see your doctor. Read more...
Sacral plexus injury. Nerves around and in the pelvis can be injured pressed by blood collection some sacral frx are right though the holes for th sacral nerve roots the sacral plexus is plastered on the front side the sacrum repeat the ct scan if getting worse. Get a wheel chair for 3 months. Read more...