Consequences of umbilical cord clamping
Stembera et al. (1963) reported measurements of
blood flow in infants immediately following birth [1].  
They found that in the period up to 100 seconds
after birth blood flow from the placenta continued at
a rate of 75 ml per minute per kilogram, then
decreased sharply.  Their results are comparable to
those of earlier researchers [2-4].  In addition to the
continuing blood flow within the first two minutes after
birth, the rates correlated with body weight, with
higher rates of blood flow to heavier babies.

In earlier research (published in German and Czech
journals) Stembera et al. found that the placenta
begins to separate from the wall of the uterus at
about 2 minutes after birth, and that the
concentration of oxygen in the umbilical arteries
begins to decline only 60 to 90 seconds after the
first breath.  They thus concluded:

    "After inclusion of all of our previous data with
    those in the present communication, it would
    appear that the first 100 seconds after birth is a
    period during which the flow and metabolic
    conditions in the maternal-placental-fetal
    system continue essentially in a manner similar
    to that in utero" [1, p573]

The work of Stembera et al. confirms White's (1773)
statement nearly 200 years earlier, that the changes
that must take place in the transition from fetal to
neonatal circulation cannot be expected to take
place within an instant of time, and at the will of a
bystander [5].

That most infants appear unharmed by immediate
clamping off of ongoing postnatal placental support
reflects the ability of the very young to recover and
heal quickly.
References
  1. Stembera ZK et al. (1963)
    Umbilical blood flow in
    healthy newborn infants
    during the first minutes after
    birth.
  2. Budin, P (1875) A quel
    moment doit-on pratiquer la
    ligature du cordon ombilical?
  3. Dunn PM. Professor Pierre
    Budin (1846-1907) of Paris,
    and modern perinatal care.
  4. Schucking, A (1877) Zur
    Physiologie der
    Nachgeburtsperiod.  
    Untersuchungen ueber den
    Placentarkreislauf nach der
    Geburt des Kindes.
  5. White C (1773) A Treatise on
    the Management of Pregnant
    and Lying-In Women.
  6. Moss AJ et al. (1963)
    Respiratory distress
    syndrome in the newborn.
    Study on the association of
    cord clamping and the
    pathogenesis of distress
    transfusion.
Full References
  1. Stembera ZK, Hodr J, Janda J. Umbilical blood flow in healthy newborn
    infants during the first minutes after birth. Am J Obstet Gynecol. 1965 Feb
    15;91:568-74.
  2. Budin, P (1875) A quel moment doit-on pratiquer la ligature du cordon
    ombilical?  Progres Medical 3:750-751, 765-767, (1876) 4:2-3.
  3. Dunn PM. Professor Pierre Budin (1846-1907) of Paris, and modern
    perinatal care. Arch Dis Child Fetal Neonatal Ed. 1995 Nov;73(3):F193-5.
  4. Schucking, A (1877) Zur Physiologie der Nachgeburtsperiod.  
    Untersuchungen ueber den Placentarkreislauf nach der Geburt des
    Kindes. Berliner Klinishe Wochenschrift 1877 #1 (1 Jan):5-7  and (8 Jan):
    18-21
  5. White C (1773) A Treatise on the Management of Pregnant and Lying-In
    Women. Canton, MA: Science History Publications, 1987, p 45 Available
    from: http://www.shpusa.com/bkindex.html
  6. Moss AJ, Duffie ER Jr, Fagan LM. Respiratory distress syndrome in the
    newborn. Study on the association of cord clamping and the pathogenesis
    of distress. JAMA. 1963 Apr 6;184:48-50.
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