Normal labour-Causes and Physiology

NORMAL LABOR (EUTOCIA):

Labor is called normal if it fulfills the following criteria.

• Spontaneous in onset and at term. 
• With vertex presentation.
• Without undue prolongation.
• Natural termination with minimal aids
• Without having any complications affecting the health of the mother

CAUSES OF ONSET OF LABOR:

Uterine distension:

• By growing fetus and liquor amnii
• Increases gap junction proteins, receptors for oxytocin and specific contraction associated proteins (CAPs).

Fetoplacental contribution:

• Onset of labor→increased CRH →increased release of ACTH →fetal adrenals →increased cortisol secretion →accelerated production of estrogen and prostaglandins from the placenta

Estrogen:

• Increases release of oxytocin
• Promotes the synthesis of myometrial receptors for oxytocin, prostaglandins & increase in gap junctions in myometrial cells.
• Accelerates lysosomal disintegration
• Stimulates the synthesis of actomyosin
• Increases the excitability of the myometrial cell membranes.

Progesterone:

• Fall in level

Prostaglandins:

• Enhance gap junction formation that initiate and maintain labor

Oxytocin and myometrial oxytocin receptors:

• Receptor number increases maximum at labor
• Receptor sensitivity increases
• Oxytocin stimulate synthesis and release of PGs (E₂ and F_2α)

Neurological factor:

• Estrogen causes α receptors and progesterone β receptors to function
• α receptors of postganglionic nerve fibers in and around cervix, and the lower part of the uterus initiate contraction

PHYSIOLOGY OF NORMAL LABOR:

UTERINE CONTRACTION IN LABOR:

• Braxton Hicks:Painless irregular involuntary sp  asmodic uterine contractions throghout pregnancy
• Waves of contractions spreads from tubal ostia downwards
• Engagement in primigravida occurs at  Beginning of labour

Pattern:

• Synchronized contraction b/w both half and upper and lower part of uterus.
• Regular pattern of wave of contraction
• Fundal dominance of contractions(10–20 seconds)
• Intensity and duration higher in upper segment of uterus
• Intra-amniotic pressure rises>20 mm Hg
• Uterine blood flow Decreases
• Relaxation phase pressure

• Uterus becomes hard & pushed anteriorly

Hypogastric pain radiating to thighs due to:

• Myometrial hypoxia
• Stretching of  peritoneum over fundus
• Stretching of cervix
• Stretching of uterine ligaments
• Compression of nerve ganglion
• Pain of uterine contractions is distributed along cutaneous nerve distribution of T10 to L1
• Pain of cervical dilatation and stretching is referred to back through the sacral plexus
• Maximum cervical dilatation during labor is 10 cm

Tonus:

• Intrauterine pressure in between contractions
• Quiescent:2–3 mm Hg
• First stage of labor:8 to 10 mm Hg

Intensity:Intrauterine pressure:190-300 Montevideo units

• 40–50 mm Hg in first stage
• 100–120 mm Hg in second stage

Duration:

• First stage:30 seconds

Frequency:

• First stage:at intervals of 10–15 minutes
• In second stage:every 2–3 minutes.

RETRACTION:

• Retraction is a phenomenon of the uterus in labor in which the muscle fibers are permanently shortened.

Effects of retraction on labor:

• Dilatation and effacement of the cervix
• Expulsion of the fetus
• Maintain the descent produced by uterine contraction
• Reduce surface area of  uterus favoring separation of placenta.
• Hemostasis after separation of placenta

PRELABOR:

Lightening:

• Prior to the onset of labor due to active pulling up of the lower pole of the uterus,presenting part sinks into the true pelvis
• Incorporation of the lower uterine segment into the wall of the uterus
• Diminishes fundal height → minimizes pressure on diaphragm→relief from mechanical cardiorespiratory embarrassment

Cervical ripening:

• Soft, 80% effaced 
• Due to the action of  PGE2
• Admits one finger easily
• Cervical canal is dilatable.

False labor pain:

• Dull
• Confined to lower abdomen 
• Relieved by enema or sedative
• Show:Expulsion of cervical mucus plug mixed with blood that represents true labour pain

Formation of “bag of waters”:

• With Dilatation of the cervical canal, lower pole of the fetal membranes becomes unsupported  & tends to bulge into cervical canal. 
• As it contains liquor, which has passed below the presentingpart, it is called bag of waters
• Certain sign of onset of labor

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