ABSTRACT
Measurements of oxytocin in maternal and fetal circulation during human labor are
reviewed and related to known changes in uterine oxytocin sensitivity during pregnancy
and labor. It is concluded that oxytocin is secreted in short-lasting spurts; therefore
levels measured with infrequent intervals do not give adequate information on the
amounts of oxytocin secreted during labor. Presently, there is little evidence for
an increased maternal secretion rate of oxytocin at the onset of labor, but during
labor a progressive increase occurs, with a maximum at the expulsive phase. Fetal
secretion rate also increases markedly during labor, but the timing of this increase
is still unknown. The dramatic increase in human uterine oxytocin receptors at term
makes the uterus responsive to very small amounts of oxytocin. Hence, an increased
oxytocin secretion rate is not a necessary prerequisite for oxytocin-stimulated contractions
during labor. Evidence from oxytocin receptor blockade and suppression of oxytocin
secretion supports the concept that oxytocin is an important stimulus for uterine
contractions in early human labor.
