eLFH - Physiology of Labour

Question 1

Stimulation of myometrium contraction

Answer 1

Contractions stimulated by:
- Calcium ions that enter myometrial cells from extracellular space

• Oxytocin increases influx of calcium ions
• Oxytocin and prostaglandins cause release of calcium from sarcoplasmic reticulum

Question 2

Role of calcium in muscle contraction

Answer 2

Calcium ions change conformational structure allowing actin and myosin to form cross links

Question 3

Change in oxytocin receptors on myometrium cell membranes during pregnancy

Answer 3

Density of cell membrane receptors for oxytocin increases throughout pregnancy

Question 4

How are coordinated, rhythmic contractions achieved by myometrium during labour

Answer 4

Gap junctions
and
Dense bands

Both increase in number during pregnancy and peak during labour

Question 5

Role of Gap junctions

Answer 5

Doughnut shaped structures that span adjacent cell membranes of myometrium cells

Allows free passage of ions including Na+

Promotes electrical connection between adjacent cells

Question 6

Role of Dense bands

Answer 6

Physically bind adjacent myometrium cells together

Adds to coordination of contractions

Question 7

Control mechanism which inhibits myometrium contraction tendency to prevent premature labour

Answer 7

Hormones including hCG interact with cell membrane receptors

Activate G proteins

Activates Adenylyl cyclase

Increase in intracellular cyclic AMP levels

Question 8

How does cyclic AMP inhibit contraction of myometrium

Answer 8

Inhibits actin-myosin cross linkage via:
Releasing active component of Protein Kinase A (PKA) and activating intracellular enzyme myosin light chain kinase

Reduces formation of gap junctions and prostaglandins by influencing gene transcription in nucleus

Question 9

Braxton Hicks contractions

Answer 9

Present when woman is aware of uterine activity but cervix has not started to dilate

Uterus always has contractile activity but just become more coordinated as pregnancy develops and there is increase in gap junctions, dense bands and oxytocin receptors and therefore woman becomes more aware of these contractions

Question 10

Labour definition

Answer 10

Uterine contractions which cause cervix to dilate

Question 11

Opposing effects of progesterone and oestrogen

Answer 11

Progesterone promotes quiescence

Oestrogen promotes excitation

Question 12

How does progesterone promote quiescence

Answer 12

Decreases gap junctions and prostaglandin synthesis

Question 13

How does oestrogen promote excitation

Answer 13

Increase oxytocin receptors, gap junctions and prostaglandin synthesis

Question 14

Initiation of labour

Answer 14

Specific stimulus unknown

cAMP pathway is down regulated in labour

Progesterone and oestrogen levels remain high (produced by placenta) but myometrium may become resistant to progesterone at end of pregnancy ?related to increased steroid production by foetus

Question 15

Stages of labour

Answer 15

First stage

Second stage

Third stage

Question 16

First stage of labour definition

Answer 16

Presence of uterine contractions leading to progressive cervical dilatation

First stage ends when cervix fully dilated

Question 17

Diameter of fully dilated cervix

Answer 17

10 cm

Question 18

How does cervix dilate

Answer 18

Softens in response to local prostaglandin production

Cervix becomes susceptible to myometrial contractions as it softens

Cervix shortens in length first (effacement)

Then aperture of cervix increases (dilatation)

Question 19

Expected rate of cervical dilatation for normal labour

Answer 19

0.5 cm per hour

Question 20

Partogram

Answer 20

Record of labour

Records:
- Maternal BP and HR
- Foetal HR
- Number of contractions per 10 mins
- Cervical dilatation

Question 21

Second stage of labour definition

Answer 21

Progressive descent of the foetus through the pelvis to achieve delivery

Occurs once cervix fully dilated

Question 22

Passive second stage of labour

Answer 22

Initially foetus descends due to uterine contraction alone

Question 23

Active second stage of labour

Answer 23

When foetus stretches vagina and perineum, woman gets strong urge to push in valsalva manoeuvre manner

Question 24

Foetal movement during second stage of labour

Answer 24

Initially foetal head lies transverse

Levator ani muscle directs so occiput moves anteriorly and neck flexes

With further descent neck extends to sweep past perineum

Then shoulders re-align so foetus moves to face laterally

Question 25

Station of foetal head definition

Answer 25

Position of foetal head in relation to ischial spines

Question 26

Duration of second stage of labour above which maternal or foetal morbidity rises

Answer 26

4 hours

Question 27

Third stage of labour definition

Answer 27

Delivery of placenta and associated membranes

Question 28

Role of uterus after baby delivers

Answer 28

Uterus has tonic contraction to immediately reduce its capacity

Placenta in incompressible so shears off inner wall of uterus

Question 29

Methods to increase tonic contraction of uterus after baby delivers

Answer 29

Mother to have skin-to-skin contact with baby
Nipple stimulation during breast feeding

Syntocinon
Ergometrine