Physiology of the birth process

Traditional teaching of the mechanisms of labour has focused on the three participants:

1 The powers.

2 The passages.

3 The passenger.

It would bedifficult to improveon this approach but before addressing these in detail it is first necessary to consider the pregnancy phase. The labour phase represents a fraction (perhaps only 1/1000th) of the total time between conception and birth. For the preceding 999 parts it is imperative that the mother is not in labour, thus ensuring that the offspring grows and develops to the appropriate extent before birth. The fetus then undergoes a complex process of maturation. Hitherto dependent almost entirely on the placenta via its umbilical lifeline for nutritional, respiratory and excretory functions, not to mention a host of other regulatory processes, the fetus must be prepared for its adaptation to extrauterine life by maturational changes in several key organ systems, notably the lungs. These processes probably occupy several weeks at the end of pregnancy.

For successful reproduction the uterus must display two fundamental qualities. It must first receive and nurture the pregnancy, and it must then launch the finished product into the world. In these two roles it must display diametrically opposite properties and to do so it has two components, very different in both structure and function - the corpus uteri and the cervix uteri. The corpus is almost entirely composed of smooth muscle -the myometrium. This must remain quiescent through almost the entire course of pregnancy before performing its contractile heroics during labour. In contrast, the cervix contains little muscle, consisting largely of connective tissue whose principal component is collagen. The collagen in the cervical stroma must retain the cervix in a firmly closed condition throughout pregnancy and then be capable of yielding during labour to allow passage of the fetus to delivery. Just as fetal maturation is a gradual process, so too is the 'maturation' which concerns the corpus and the cervix, and it seems clear that the complex endocrine and other changes which 'mature' both the fetus and the uterus are, in normal conditions, intimately linked.

While labour proper is generally a process lasting a few hours, its onset, far from being sudden, is the culmination of a gradual process which has been evolving over several weeks. This development phase of preparation for parturition has been suitably entitled prelabour [1].

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Once your pregnancy is over and done with, your baby is happily in your arms, and youre headed back home from the hospital, youll begin to realize that things have only just begun. Over the next few days, weeks, and months, youre going to increasingly notice that your entire life has changed in more ways than you could ever imagine.

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