Stages of Normal Labour

Stages of Normal Labour


INTRODUCTION:

  • PARTURITION is defined as the process of bringing forth of young which comprises of multiple transformations in both uterine and cervical functions

There are four phases :

  • Quiescence
  • Activation phase
  • Stimulation phase
  • Involution phase

FIRST STAGE OF LABOUR:

Following are the major events during labour:

  • Gradually increasing uterine contractions
  • Retraction
  • Dilatation of cervix
  • Effacement of cervix
  • Lower uterine segment formation

DILATATION OF THE CERVIX:

  • Structural components of the cervix
  • Smooth muscle (5–20%)
  • Collagen 
  • Ground substance.

Predisposing factors:

  • Softening of cervix
  • Fibromusculoglandular hypertrophy
  • Increased vascularity
  • Accumulation of fluid in between collagen fibers
  • Breaking down of collagen fibrils by enzymes collagenase and elastase
  • Change in GAGs

FACTOR RESPONSIBLE:

Uterine contraction and retraction:

  • Polarity of uterus i.e. co-ordination between fundal contraction and cervical dilatation.
  • Upper segment contracts→ lower segment and cervix dilate

Fetal axis pressure:

  • Contractions of the circular muscle of uterus→fetal podalic pole→vertebral column→well-fitted fetal head→stretching of lower segment →dilatation of  cervix

Bag of membranes:

  • Uterine contractions→ hydrostatic pressure in forewaters→dilatation of  cervix

Vis-a-tergo:

  • Downward thrust of the presenting part of the fetus and upward pull of the cervix over the lower segment

EFFACEMENT:

  • Muscles of cervix are pulled upward and merges with lower uterine segment.
  • Primigravidae:effacement precedes  cervix dilatation
  • Multiparae:effacement & cervix dilatation occurs simultaneously
  • Lead to expulsion of mucus plug

LOWER UTERINE SEGMENT:

Anatomical Features :

  • Developed from the isthmus of uterus

Boundries:

  • Above:Physiological retraction ring(in labor)
  • Below:Fibromuscular junction of cervix and uterus(in labor)
  • Anteriorly:peritoneum
  • Size and Shape:7.5–10 cm,cylindrical 
  • Poor retractile property

Thining of wall:

  • Relaxation of muscle fibers to allow elongation
  • Contraction and retraction during labor
  • Descent of the presenting part

Clinical Significance:

  • Receptive relaxation→ formation of complete birth canal along with fully dilated cervix→expulsion of the fetus
  • Through this segment cesarean section is done
  • Implantation of placenta in lower segment 
  • Poor decidual reaction in this segment facilitates morbid adherent placenta
  • Obstructed labor, the lower segment is very much stretched and thinned out and ruptures
  • Postpartum hemorrhage if placenta is implanted over

EVENTS IN SECOND STAGE OF LABOR:

2 stages:

  • Propulsive: From full dilatation until head touches the pelvic floor
  • Expulsive:Since the time mother has irresistible desire to “bear down” and push until the baby is delivered.
  • Latent phase of labour is followed by Accelerated phase

MECHANISM:

Full dilatation of the cervix          Stronger uterine contraction and retraction
            ↓                                                       ↓
Membranes ruptures                       Contractions of the circular muscle fibers  
            ↓                                                       ↓
↓volume of uterine cavity                 Uterus elongate during contraction
                                                                     ↓
                                                     ↓anteroposterior and transverse diameters

FORCES:

  • Downward:By uterine contractions supplemented by voluntary contraction of abdominal muscles
  • Upward:Elastic recoil of the tissue of the vagina ,pelvic floor, bony and soft tissues of birth canal.
  • Counterbalanced by the power of retraction.
  • Increasing contraction and retraction, the upper segment becomes more  thicker with thinning of lower segment.
  • The expulsive force of uterine contractions is added by voluntary contraction of the abdominal muscles  called “bearing down” efforts.

EVENTS IN THIRD STAGE OF LABOR:

  • Birth of infant to delivery of placenta is 3rd stage of labour.
  • Placental separation
  • Placental Descent to the lower segment
  • Placental Expulsion with the membranes

PLACENTAL SEPERATION:

  • Retraction→ ↓surface area at placental site to its half→ buckling
  • Shearing force between placenta and the placental site which brings ultimate separation

  • Two ways of separation
  • Central separation (Schultz):Detachment starts at center leads to retroplacental hematoma
  • Marginal separation (Mathews-Duncan):starts at the margin & more frequently.

SEPARATION OF THE MEMBRANES:

  • Loosely attached  membranes(to active part) thrown into multiple folds
  • Attached to the lower segment are already separated during its stretching.
  • separation is by uterine contraction and weight of the placenta

EXPULSION OF PLACENTA:

  • Contraction and retraction of uterus→ forced down placenta to flabby lower uterine segment or upper part of the vagina
  • Voluntary contraction of abdominal muscles (bearing down efforts) or by manual procedure expelles placenta out.
  • CCF is most likely in pregnancy at 3rd stage of labour
  • Pressure inside uterus in 3rd stage is  100-120 mm Hg.
Exam Question
 
  • Latent phase of labour is followed by Accelerated phase
  • CCF is most likely in pregnancy at 3rd stage of labour
  • Second stage of labor starts from Full dilatation of cervix
  • Expulsion of the fetus, Increase in contraction & Cervical dilatation are  seen in 2nd stage of labor
  • From full dilatation of cervix to complete birth of baby is 2nd stage
  • Birth of infant to delivery of placenta is 3rd stage of labour.
  • Pressure inside uterus in 3rd stage is  100-120 mm Hg.
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