Purpose The extensor pollicis longus (EPL) and brevis (EPB) and abductor pollicis longus (APL) are muscles located in the posterior compartment of the forearm. These muscles allow the thumb to move independently from the other four fingers by attaching to the thumb separately. This study's goal was to investigate the tendon variations and insertions of these muscles. Methods Our study was conducted on 43 fetal cadavers (86 upper extremities) aged between 17 and 40 weeks of gestation. This study investigated the tendon numbers and insertions of the EPL, EPB, and APL. Results The tendon numbers of the EPL and EPB were observed to range between 1 and 2, and the tendon numbers of the APL ranged between 1 and 5. The EPL was found to insert into the distal phalanx via the dorsal aponeurosis in all extremities. It was observed that the EPB tendons inserted into the proximal phalanx, distal phalanx, and dorsal aponeurosis. It was determined that the APL tendons inserted into the basis of the first metacarpal, abductor pollicis brevis, opponens pollicis, and trapezium bone. Conclusion Knowing the tendon variations and insertions of the EPL, EPB, and APL muscles during the fetal period will be useful in planning treatments to correct the congenital thumb anomalies and the loss of function after injury.