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 Important
- 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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