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This Is Thought To Be Mainly A Physical Phenomenon As The Uterus Is Capable ...

This is thought to be mainly a physical phenomenon as the uterus is capable of contraction, whereas the placenta (being devoid of muscular tissue) is not. We should note the characteristic of the myometrium which is unique in the animal kingdom, and this is the ability of the myometrial fibres to maintain its shortened length after each contraction and then to be able to contract further with subsequent contractions. This characteristic results in a progressive and (normally) fairy rapid reduction in the overall surface area of the placental site. (Sanborn B M et al. 1998)

In the words of Rogers (J et al. 1998), by this mechanism the placenta is undermined, detached, and propelled into the lower uterine segment.

Other physiological mechanisms also come into play in this stage of labour. Placental separation also occurs by virtue of the physical separation engendered by the formation of a sub-placental haematoma. This is brought about by the dual mechanisms of venous occlusion and vascular rupture of the arterioles and capillaries in the placental bed and is secondary to the uterine contractions (Sharma J B et al. 2005). The physiology of the normal control of this phenomenon is both unique and complex. The structure of the uterine side of the placental bed is a latticework of arterioles that spiral around and inbetween the meshwork of interlacing and interlocking myometrial fibrils. As the myometrial fibres progressively shorten, they effectively actively constrict the arterioles by kinking them . Baskett (T F 1999) refers to this action and structure as the living ligatures and physiologic sutures of the uterus.

These dramatic effects are triggered and mediated by a number of mechanisms. The actual definitive trigger for labour is still a matter of active debate, but we can observe that the myometrium becomes significantly more sensitive to oxytocin towards the end of the pregnancy and the amounts of oxytocin produced by the posterior pituitary glad increase dramatically just before the onset of labour. (Gülmezoglu A M et al. 2001)

It is known that the F-series, and some other) prostaglandins are equally active and may have a role to play in the genesis of labour. (Gulmezoglu A M et al. 2004)
>From an interventional point of view, we note that a number of synthetic ergot alkaloids are also capable of causing sustained uterine contractions. (Elbourne D R et al. 2002)

chapter 2 discuss active management, criteria, implications for mother and fetus.
This essay is asking us to consider the essential differences between active management and passive management of the third stage of labour. In this segment we shall discuss the principles of active management and contrast them with the principles of passive management.

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