Chapter 6. Origins of labor onset

6.1. MECHANISMS OF UTERINE CONTRACTIVE ACTIVITY

Labor is a process completing the pregnancy; it consists in expulsion of the fetus and afterbirth from the uterine cavity through the birth canal ( after the fetus has attained viability, after 22 weeks gestation).

Labor is a complex physiological genetically programmed process, an unconditioned reflex whose regulation is provided by virtually all organs and systems in the female body. For various reasons pregnancy can be terminated at any stage. The fetus can only survive once it has acquired certain anatomical and functional developmental features that depend on multiple conditions.

Until 22 weeks gestation the fetus is not viable. Termination of pregnancy prior to 22 weeks gestation is termed abortion rather than delivery.

Labor at term, normal labor and physiological delivery is considered as such when it occurs at an age of 37 to 42 weeks, and a mature fetus is born whose adaptation to external conditions is easy and fast.

A woman who gives birth for the first time is referred to as primiparous, the one who has experienced one or more previous childbirths - multiparous. During labor the woman giving birth is commonly referred to as parturient woman.

6.2. ONSET OF LABOR AND REGULATION OF UTERINE

ACTIVITY

The factors triggering the onset of labor and regulating the uterine activity have not been studied well enough yet. Multiple theories were put forward to shed light on the mechanism of labor onset: the theory of foreign body, the mechanical, placental, immune and other theories; most of them are of merely historical interest now.

In 1940-1950s investigations undertaken by Russian scientists broke new ground as to the onset of labor and the way it is regulated; they substantiated the theory of neurohumoral regulation of the uterine function including labor (A.P. Nikolaev, A.I. Petchenko, L.S. Persianinov, N.L. Garmasheva, I.I. Jakovlev and others). According to this theory, the onset of labor is associated with changes taking place in the CNS and the blood of the pregnant woman. The leading role here is played by the hypothalamus, pituitary gland, and the limbic system (amygdoid bodies, etc.). The ovaries, placenta and other glands of internal secretion have a regulatory effect on the uterine motor function, too.