Introduction of more obstetric clamps
    More articles appeared in the 1930s describing new obstetric clamps [1-5].  Among
    these is one article recommending use of a rubber band for tying the cord [6].  
    Thus clamping the cord was becoming more common, and it appears the clamps
    were applied earlier and earlier following birth.

Umbilical cord blood banking
    Clamping the cord sooner after birth resulted in more residual placental blood,
    which was recognized in several articles as worth storing for later use for
    transfusions [7-11].

Research on postnatal placental circulation
    Research continued on the advantage to the newborn of waiting for placental
    circulation to continue in the neonatal period [12, 13]

(in progress)
References
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  1. Ryder GH. Presentation of instruments: (1) A uterine packer for cesarean
    sections; (2) An umbilical cord clamp. Am J Obstet Gynecol 1932; 23:438.
  2. Kane HF. An umbilical cord clamp. Am J Obstet Gynecol 1934; 27:623-4.
  3. Kahn ME. An improved umbilical cord clamp. Am J Obstet Gynecol 1936;
    513-4, 903.
  4. Ziegler CE. A new all metal umbilical cord clamp. Am J Obstet Gynecol
    1936; 884-6.
  5. White MR. Umbilical cord clamp. Am J Obstet Gynecol 1939; 37:345.
  6. Carrington GL. The use of a rubber band for tying the umbilical cord. Am
    J Obstet Gynecol 1937; 698.
  7. Goodall JR, Anderson FO, Altimas GT, MacPhail FL (1938) An
    inexhaustible source of blood for transfusion and its preservation.  
    Surgery, Gynecology and Obstetrics 66:176-178.
  8. Barton FE, Hearne TM (1939) The use of placental blood for transfusion.
    JAMA 113:1475-1478.
  9. Grodberg BC, Cary EL (1939) A study of seventy-five transfusions with
    placental blood.  New England Journal of Medicine 219:471-474.
  10. Halbrecht J. (1939) Transfusion with placental blood. Lancet 2:202-203.
  11. Halbrecht J (1939) Fresh and stored placental blood. Lancet 2:1263-
    1265.
  12. Allmeling A. Die Gewichtszunahme von Neugeborenen infolge
    postnataler Transfusion. Zentralbl Gynakol 1930; 54:850-860.
    (Zentralblatt für Gynäkologie).
  13. Frischkorn HB , Rucker MP. The relationship of the time of ligation of the
    cord to the red blood count of the infant. Am J Ostet Gynecol 1939; 38:
    592-594.