Labour - Mechanism of labour

Question 1

Define labor, term, preterm labor, miscarriage, prolonged labor

Answer 1

Definitions:
 labor: products of gestation expelled from uterine cavity after 24w
 term: labor at 37-41+6 weeks
 preterm labor: labor at 24-37w
 miscarriage: labor before 24w resulting in non-viable fetus
 prolonged labor: labor lasting ≥24h in primigravida and ≥16h in multigravidas

Question 2

Determinants of labor

Reasons for why successful vaginal delivery cannot be predicted accurately

Answer 2

3Ps of labor: main determinants of labor
- passage: birth canal (bony pelvis, soft tissue, cervix)
- passenger: fetus (size, presentation, position)
- power: uterine contractions, maternal effort

1. Passenger
   - Size difficult to predict accurately before birth
   - Reserve difficult to predict
   - Degree of flexion of fetal head (attitude) can change
   - Depend on position
   - Degree of safe moulding not predictable
2. Passage
   - Size difficult to assess
   - Need to take shape into account
3. Power
   - Can be augmented but response of uterus to stimulation not predictable
   - Maternal effort not predictable

Question 3

Stages of labor

Answer 3

Question 4

Onset of labor

Definition
Clinical features

Answer 4

Question 5

Labor

Signs of labor

Answer 5

Question 6

Labor

Mechanism initiating labor

Answer 6

Question 7

Uterine contraction

• Physiology
• Monitoring

Answer 7

Question 8

Female pelvis inlet, mid-pelvis and outlet diameter

Answer 8

Pelvic inlet: AP diameter (11cm) < transverse diameter (13.5cm), i.e. a horizontal strait

Mid-pelvis: AP diameter (12cm) = transverse diameter, i.e. almost circular
- interspinous diameter and AP diameter can be assessed clinically

Pelvic outlet: AP diameter (13.5cm) > transverse diameter (11cm), i.e. a sagittal strait
- intertuberous diameter = transverse diameter of pelvimeter can be assessed clinically

Question 9

Fetal head

• Moulding staging
• Caput cause and clinical implication

Answer 9

Question 10

Fetal head

• Attitude and diameter

Answer 10

Question 11

Terminologies to describe fetal position

Answer 11

Lie: relationship between long axes of fetus and uterus
- term: longitudinal (0°), oblique (45°), transverse (90°)

Presentation: which part is presented to the pelvis
- cephalic: head (majority) → can be divided into vertex, brow and face depending on attitude
- breech (podalic): bottom
- shoulder: for transverse lie

Attitude: degree of flexion of fetal upper C-spine
- well-flexed → vertex presentation (most ideal), i.e. suboccipito-bregmatic (9.5cm)
- less well-flexed → occiptofrontal (11.5cm)
- extended → brow presentation, i.e. occipitomental (13cm) [vaginal delivery C/I]
- hyper-extended → face presentation, i.e. submentobregmatic (9.5cm)

Position: relationship of denominator to inlet of maternal pelvis

Station and engagement:
- engagement: when greatest transverse diameter (biparietal) has passed through inlet of true pelvis (i.e. ≤2/5 head palpable above brim)
- station: how many fifths of head palpable

Descent (station): position of presenting part with regards to ischial spine (e.g. -1 = 1cm above spine, +1 = 1cm below spine)

Question 12

Mechanism of second stage of labor

Answer 12

Question 13

3rd stage of labor

Signs of placental separation

Timing

Answer 13

Question 14

Summarize all stages of labor

Answer 14