Theories and causes of onset of labour

Theories and causes of onset of labour

Labor is a coordinated series of involuntary and intermittent contractions of the uterus. These contractions play a crucial role in expelling the fetus and placenta from the mother’s body. This process is facilitated by the combination of uterine contractions and pressure exerted by the abdominal muscles, which work together during the expulsion phase of delivery. As contractions occur regularly, the cervix gradually becomes thinner and more open, a process known as effacement and dilation. The application of sufficient abdominal pressure aids in pushing the baby out of the mother’s womb. Labor and delivery require a woman to utilize her coping methods psychologically and physiologically. Normally, labour begins when the fetus reaches a mature age (38-42 weeks’ age of gestation).     

This is to ensure survival of the fetus with the extrauterine life. The mechanism that converts Braxton Hicks Contractions (painless contractions) to strong and coordinated uterine contractions is unknown.

In some cases, labour occurs before the fetus reaches the mature age (preterm birth) while in others it is delayed (post term birth). Although the exact mechanism that initiates labour is unknown.

Theories have been proposed to explain how and why labour occurs.

Uterine Stretch theory

The concept is founded upon the principle that any hollow organ within the body, when extended to its maximum capacity, will inherently undergo contractions to expel its contents. The uterus, being a hollow and muscular organ, experiences stretching as a result of the developing fetal structures. Consequently, this stretching generates increases pressure, leading to physiological responses (uterine contractions) that initiate for the onset of labour.

Oxytocin theory

Pressure on the cervix stimulates the hypophysis  (pituitary gland) to release oxytocin from the maternal posterior pituitary gland. As pregnancy advances, the uterus becomes more sensitive to oxytocin. Presence of this hormone causes the initiation of contraction of the smooth muscles of the body (uterus is composed of smooth muscles).

Progesterone deprivation theory

Progesterone is the hormone designed to promote pregnancy. It is believed that presence of this hormone inhibits uterine motility. As pregnancy advances, changes in the relative effects estrogen and progesterone encourage the onset of labour. A marked increase in estrogen level is noted in relation to progesterone, making the latter hormone less effective in controlling rhythmic uterine contractions. Also, in later pregnancy, rising fetal cortisol levels inhibit progesterone production from the placenta. Reduce progesterone formation initiates labour.

Prostaglandin theory

The Prostaglandin theory suggests that during the later stages of pregnancy, there is an increases in prostaglandin levels within the fetal membranes and uterine decidua. This particular hormone is discharged from the lower region of the fetal membrane, commonly referred to as the fore-bag. Furthermore, a reduction in the quantity of progesterone further contributes to the elevation of prostaglandin levels. Consequently, the synthesis of prostaglandin prompts contractions of the uterus, thereby initiating the process of labour.

Theory of Aging Placenta

As the placenta advances in age, it diminishes the blood supply to the uterus. This occurrence acts as a trigger for uterine contractions, thus initiating the onset of labour.

Causes of onset of labour

The precise mechanism responsible for the initiation or onset of labour is still remains unknown. However, two proposed causes are the endocrine pathway and the mechanical stretch pathway.

Mechanical factors (uterine distention)

Stretching effect on endometrium is caused by the growing fetus and liquor amnii. This  stretching of endometrium lead to progressive uterine distention and increases intrauterine pressure, which causes the myometrial irritability then cause the onset of labour.

Estrogen: Increase release of oxytocin from the pituitary gland. Promotes the synthesis of receptors for oxytocin in the myometrium and decidua.

Progesterone: Production of dehydroepiandrosterone sulphate (DHEA-S) and cortisol inhibits the conversion of fetal pregnenolone to progesterone levels therefore fall before labour. This is the alteration in the estorgen progesterone ratio rather than the fall in the absolute concentration of progesterone which is linked with prostaglandin synthesis.

Prostaglandins: Important factors which initiate and maintain labour. Major sites of synthesis are amnion, chorion, decidual cells , myometrium. Synthesis is triggers by rise in estrogen level, glucocorcoid, mechanical stretching in late pregnancy, increase cytokines infection, vaginal examination,  separation/rupture of the membranes. Prostaglandin synthesis reaches peak during the birth of placenta probably contributing to its expulsion and to the control of PPH.

Oxytocin – receptors are increased in the uterus with the onset of labour. It promotes the release of prostaglandins from the deciduas. Oxytocin synthesis is increased in the deciduas and in the placenta. Vaginal examination and amniotomy cause rise in maternal plasma oxytocin level. Oxytocin level reaches the maximum at the movement at birth.

Neurological factors

Labour may also be initiated through nerve pathways.  Both α and β adrenergic receptors are present in the myometrium. The contractile are response is initiated through the α receptors of the postganglionic nerve fibers in and around the cervix and lower part of the uterus.

Factors of labour

Labour entails the interaction of the so-called 5Ps:

1. Passageway (birth canal)
2. Passenger ( fetus and placenta)
3. Power (contractions)
4. Psychological response
5. Position (maternal)

1. Passageway

It consist of maternal pelvis and soft tissue

• Bony pelvis: – False pelvis and True pelvis
• Pelvic inlet, Mid pelvis, Outlet
• Pelvic shape: Anthropoid , Android, Gynecoid, Platypelloid
• The soft tissue
• The cervix (effacement and dilatation)

2. Passenger

• Fetal skull: is the largest presenting part and least compressible fetal structure,
• Passenger sutures -inter-membranous spaces that allow molding.
• Passenger fontanels
•  Fetal attitude – posturing (flexion or extension )
•  Fetal lie-  transverse or oblique

3. Power

Uterine contraction involuntary and voluntary power combines to expel the fetus and placenta from the uterus.

4. Psychological responses:

The birth experience influence the woman’s self confidence, self esteem, and her view of life, her relationships, and her children.

Factors influencing a positive birth experience include:

• clear information on procedure
• positive support, not being alone
• sense of mastery, self- confidence
• trust in staff caring for her
• positive reaction to the pregnancy
• personal control over breathing
• preparation for childbirth experience.

5. Position (Maternal)

Changing positions and moving around during birth offer several benefits, it facilitate fetal descend and rotation. Squatting position enlarges pelvic outlet by approximately 25% . The use of upright or lateral position compared with supine or lithotomy positions may:

• reduce the duration of 2^nd  stage of labour
• reduce number of assisted deliveries