Numbness in the lateral thighs during pregnancy is usually caused by compression of the lateral femoral nerve.
The lateral femoral cutaneous nerve runs through the lateral border of the psoas major muscle to the groin, crossing the inguinal fascia. During pregnancy, the physiological structures such as the pelvis change in angle and tend to compress the lateral femoral cutaneous nerve, resulting in numbness in the lateral thighs, which often disappears with the end of pregnancy.
Another cause of numbness in the lateral thighs during pregnancy is a herniated lumbar disc, which increases pressure on the disc as the fetus grows, further compressing the nerve and causing numbness in the thighs, which can be relieved at the end of pregnancy, or may persist but to a lesser extent.