Pregnancy is a physiological state which subjects the female body to a range of extreme change in form and function. One of these is in the level of oxytocin.  Oxytocin is a small peptide hormone, made up of 9 amino acids, secreted by the hypothalamus and passing into the posterior pituitary gland, and thence into the bloodstream where it acts on various target organs such as the breasts, the uterus, on various erectile tissues, and directly on other neurons in the brain. Oxytocin is also synthesized outside the brain, in the ovary, testis, pancreas and adrenal gland.

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Oxytocin levels have been measured in pregnancy and labor but with widely mixed results. Some studies show a slow increase in oxytocin during pregnancy, with the highest level at term. In other studies, on primate species and in cows, it has been observed to rise to four times the original level on the day of parturition. This may be highest just before the delivery of the head. Yet another has shown that levels keep increasing until 36 weeks when they peak. Another group observed no significant variation at any period of pregnancy. This wide difference in results may be due to several factors:

• The short half-life of oxytocin, only a few minutes, as compared to other hormones, can make measurements inaccurate
• The pulsatile secretion of oxytocin lends itself to great variation in measured levels

Overall, studies show a rise in oxytocin levels from late pregnancy onwards, to reach their zenith as labor starts and especially during the delivery phase itself. However, some animal studies have shown that even without oxytocin action, labor can start and proceed apparently normally, and the delivery is also unaffected. Such animals though, were observed to have lactation problems.

The Role of Oxytocin in PregnancyEven as they rise, oxytocin levels peak at night. This is associated with increasing uterine activity at night.

Oxytocin was discovered as a uterotonic agent at the very beginning and is responsible for facilitating forceful and coordinated uterine contractions to expel the fetus at parturition. However, synthetic oxytocin use to augment or induce labor has not been associated with uniformly good results and needs more study to identify and quantify the harmful effects, which may include hyperstimulation syndromes, fetal distress, uterine rupture, meconium staining, and even autism in the offspring.

Oxytocin in the Postpartum Period

Postpartum depression has been considered to be a fallout of inadequate oxytocin secretion in pregnancy and the postpartum period. One study has shown an increase in levels between the 35th week of pregnancy to 6 months postpartum, but this was not seen in women with symptoms of postpartum depression. In this group oxytocin levels actually decreased from the 38th week to the second postpartum day. This was linked also to a history of difficult lactation and previous episodes of depression.

Oxytocin and the Circadian Rhythm

Another set of interesting findings regarding oxytocin in pregnancy is that:

• Oxytocin secretion occurs in a pattern which is fixed early in pregnancy and is independent of light exposure. Thus it remains constant despite night shift work or travel across time zones. The circadian pacemaker for oxytocin is thus different from the clock that regulates other body functions. Even though the hypothalamus, where oxytocin is synthesized, is also the recipient of other light-associated endocrine peptides from the suprachiasmatic nucleus, they do not affect oxytocin secretion in pregnancy.
• The circadian pattern of secretion is what determines the timing of labor and birth in relation to the circadian rhythms, and not light exposure or other light-related cues.
• In oxytocin deficient individuals, circadian rhythms surrounding the events of birth are lost and labor is much more likely to become abnormal. This is even more important as early in pregnancy, oxytocin maintains the corpus luteum and thus delays labor onset, but later in pregnancy it augments uterine contractions. This means that circadian rhythms are crucial in promoting changes in sensitivity to oxytocin in peripheral organs, which lead to precise timing of labor changes.

Reviewed by Gillian D’Souza, MSc