Physiology Of The Second Stage Of Labour

Question 1

What is the second stage of labour?

Answer 1

• it is from full dilatation of the cervix to the birth of the baby

Question 2

What are the signs of the transitional phase?

Answer 2

• maternal restlessness
• desire for pain relief
• contractions become stronger and longer but may be less frequent
• increasingly difficult to hold a conversation
• increasing vocalisation
• involuntary bearing down

Question 3

Describe the uterine action during the second stage

Answer 3

• contractions become stronger and longer but may be less frequent, allowing both mother and fetus recovery periods
• the membranes often rupture spontaneously towards the end of the 2nd stage
  - -> the consequent drainage of liquor allows the pp to be directly applied to the vaginal tissues and aid the process of distension
• fetal axis pressure increases flexion of the pp resulting in smaller presenting diameters, more rapid progress and less trauma to both mother and fetus
• the contractions become expulsive as the fetus descends further into the vagina
• pressure from the pp stimulates nerve receptors in the pelvic floor (Ferguson reflex)
  - -> as a consequence the woman experiences the need to push
  - -> this reflex may be initially controlled to a limited extent but becomes increasingly compulsive, overwhelming and involuntary
  - -> the mothers response is to employ her secondary powers of expulsion by contracting her abdominal muscles and diaphragm

Question 4

Describe how the soft tissues are displaced during the second stage

Answer 4

• as the fetal head descends, the soft tissues of the pelvis become displaced
• anteriorly the bladder is pushed upwards into the abdomen where it is at less risk of injury during fetal descent
  - -> this results in the stretching and thinning of the urethra so that its lumen is reduced
• posteriorly the rectum becomes flattened into the sacral curve no the pressure of the advancing head expels any faecal matter
• the levator ani muscles dilate, thin out and are displaced laterally and the perineal body is flattened, stretched and thinned out
• the fetal head becomes visible at the vulva, advancing with each contraction and receding between contractions until crowning takes place
• the head is then born
• the shoulders and body follow with the next contraction accompanied by a gush of amniotic fluid and sometimes of blood
• birth of baby

Question 5

What are the signs of the second stage of labour?

Answer 5

• some women feel a strong desire to push before full dilatation occurs
• rupture of the forewaters
• dilatation and gaping of the anus
• anal cleft line (purple line)
  - -> pigmented mark in the cleft of the buttocks which gradually ascends the anal cleft as the labour progresses
• appearance of the rhomboid of Michaelis
  - -> kite shaped area of bone moves backwards as it pushes the wings of ilea out increase the diameters of the pelvis
• show
  - -> loss of blood-stained mucus which often accompanies rapid dilatation of the cervical os towards the end of the 1st stage

Question 6

Describe the latent phase of the 2nd stage

Answer 6

• in some women, full dilatation of the cervical os is recorded but the pp may not yet have reached the pelvic outlet
• no urge to bear down
• passive descent should be allowed until pp is visible
• muscle fibres of uterine wall need to shorten and thicken further
• stretch receptors in vagina, rectum and perineum communicate changes in volume, tension and tone

Question 7

Describe the active phase of the 2nd stage

Answer 7

• urge to push allows pp to descend to compress the tissues of the pelvic floor
• 1cm above the ischia spines nerve receptors are stimulated in pelvic floor (ferguson’s reflex) and an uncontrollable urge to push is experienced
• expulsive contractions - muscles fibres of vagina and uterus draw up an tighten to provide a taut surface
• steep rise in catecholamines during last minutes before birth, woman is alert (fetal ejection reflex)

Question 8

Describe the mechanism of normal labour

Answer 8

• the fetus normally engages in the occipito transverse position as this is the widest diameter at the pelvic inlet and descent occurs
• flexion -> due to the pressure from contractions along the long axis of the fetal pole this encourages flexion of the fetal head (encourages SOB)
• internal rotation -> the pp hits the pelvic floor, it will rotate forwards to lie underneath the symphysis pubis
• once crowning has occurred the head can extend, pivoting around the pubic bone
  - -> this releases the sinciput, face and chin which sweep the perineum and are born by moment of extension
• restitution -> the twist in the neck from internal rotation is now corrected, the occiput moves 1/8 of a circle towards the side from which it started
• external rotation -> the shoulders rotate into the AP diameter
• lateral flexion -> anterior shoulder slips beneath the sub-pubic arch and the posterior shoulder passes over the perineum
• the remainder of the body is born by lateral flexion as the spine bends sideways through the curved birth canal