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(dansereau J Et Al. 1999). the High Progesterone Levels Encountered In ...

(Dansereau J et al. 1999).

The high progesterone levels encountered in pregnancy are also relevant insofar as they tend to reduce the general vascular tone thereby increase venous pooling. This, in turn, reduces the venous return to the heart and this would (if not compensated for by the increased blood volume) lead to hypotension which would contribute to reductions in levels of foetal oxygenation. (Baskett T F 1999). Coincident and concurrent with these heamodynamic changes are a number of physiological changes in the coagulation system.
There is seen to be a sharp increase in the quantity of most of the clotting factors in the blood and a functional decrease in the fibrinolytic activity. (Carroli G et al. 2002). Platelet levels are observed to fall. This is thought to be due to a combination of factors. Haemodilution is one and a low level increase in platelet utilisation is also thought to be relevant. The overall functioning of the platelet system is rarely affected. All of these changes are mediated by the dramatic increase in the levels of circulating oestrogen. The relevance of these considerations is clear when we consider that one of the main hazards facing the mother during the third stage of labour is that of haemorrhage. (Soltani H et al. 2005) and the changes in the haemodynamics are largely germinal to this fact.

The other major factor in our considerations is the efficiency of the haemostasis produced by the uterine contraction in the third stage of labour. The prime agent in the immediate control of blood loss after separation of the placenta, is uterine contraction which can exert a physical pressure on the arterioles to reduce immediate blood loss. Clot formation and the resultant fibrin deposition, although they occur rapidly, only become functional after the coagulation cascade has triggered off and progressed. Once operative however, this secondary mechanism becomes dominant in securing haemostasis in the days following delivery. (Sleep, 1993).

The uterus both grows and enlarges as pregnancy progresses under the primary influence of oestrogen. The organ itself changes from a non-gravid weight of about 70g and cavity volume of about 10 ml. to a fully gravid weight of about 1.1 kg. and a cavity capacity of about 5 litres. This growth, together with the subsequent growth of the feto-placental unit is fed by the increased blood volume and blood flow through the uterus which, at term, is estimated to be about 5-800 ml/min or approximately 10-15% of the total cardiac output
(Thilaganathan B et al. 1993). It can therefore be appreciated why haemorrhage is a significant potential danger in the third stage of labour with potentially 15% of the cardiac output being directed towards a raw placental bed.

The physiology of the third stage of labour also involves the mechanism of placental expulsion.

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