Purpose We aimed to describe the distribution of the motor branches of the median and ulnar nerves that innervate the superficial flexor muscles in detail, as well as to determine any communication between these two nerves in a series of 100 human fetuses. Methods This study was performed on 200 upper limbs from 100 fetuses. However, the motor branches of the median nerve were determined on 50 upper limbs because of the developmental properties of medial epicondylar muscles. Results The motor branch, which innervates the pronator teres, is classified into two types and four subtypes. The flexor carpi radialis branch arose as a single branch in 30% of the cases. The innervation of the flexor digitorum superficialis was also performed by a single muscular branch in 88% of the cases and by two branches in 12% of the cases. The ulnar nerve was classified into two types according to the number of muscular branches in the forearm. Martin-Gruber anastomosis was observed in 7.5% of the cases. Conclusions These results show differences from classical definitions regarding the muscular branching patterns of the median and ulnar nerves. We suggest revisiting the classical descriptions of innervation patterns of pronator teres, palmaris longus and flexor carpi radialis muscles, since the variations observed in their innervation patterns are more diverse than has been described. © Springer-Verlag 2009.