Lecture 18: Labour and delivery Flashcards
What is parturition?
Expulsion of the products of conception (fetus and placenta)
Labour: when this occurs after 24 weeks
Miscarriage: <24 weeks
Labour before the 37th week= premature/pre-term labour
What does the explusion of the fetus require?
- creation of a birth canal
- release of the structures which normally retain the fetus in utero
- enlargement and realignment of cervix and vagina
- expulsion of fetus
- expulsion of placenta and changes to minimise blood loss from mother
When does the uterus become palpable?
12 weeks
What is the ‘lie’ of the fetus?
Relationship of the long axis of the fetus to the long axis of the uterus
- commonest lie is longitudinal with head/buttock posterior
- fetus normally has a flexed attitude
What is the ‘presentation’ of the fetus?
Describes which part of the fetus is adjacent to the pelvic inlet
- if baby lies longitudinally the presenting part may be the head (cephalic) or the breech (podalic)
e. g. longitudinal lie, vertex presentation
e. g. longitudinal lie, breech presentation
e. g. transverse lie, shoulder presentation
What is the usual diameter of presentation?
- 5 cm (when baby is longitudinal, cephalic position and well flexed so the vertex presents to the pelvic inlet)
- birth canal therefore needs to be 10cm for fetus to pass through
What is the pelvic inlet bouded by?
Posteriorly: sacral promontory
Laterally: ilio-pectinal line
Anteriorly: superior pubic rami and upper margin of pubic symphysis
-true diameter size of this is 11cm (softening of pelvic ligaments may allow some expansion)
(birth canal diametre can’t extend beyond the limits determined by the pelvis)
How is the fetus normally retained in the uterus?
- cervix
- inactive myometrium
What must the cervix do in order to create a birth canal?
-dilate (facilitated by structural changes known as cervical ripening)
-retract anteriorly
(at some time during this process the fetal membranes rupture releasing amniotic fluid)
What happens in cervical ripening/softening?
Cervix has high connective tissue content made up of collagen fibres embedded in proteoglycan matrix
- ripening involves reduction on collagen and marked increase in glycosaminoglycans (GAGs) which decrease aggregration of collagen fibres, so collagen bundles loosen
- also influx of inflammatory cells and an increase in nitric oxide output (triggered by prostaglandins E2 and F2alpha)
What happens to the myometrium?
Myometrium is made up of bundles of smooth muscle cells
- during pregnancy the myometrium gets much thicker due to increase cell size and glycogen deposition
- an intracellular apparatus containing actin and myosin, triggered by a rise in intracellular calcium generate force
- rise in calcium conc is produced by AP’s in cell membrane
- AP’s spread from cell to cell via gap junctions allowing coordinated contraction over myometrium
- some smooth muscle cells are capable of spontaneous depolarization and AP generation so can act as pacemakers
Is the myometrium always motile?
Yes it is always spontaneously motile
-in early pregnancy contractions may occur every 30 mins, but are of low amplitude
-as pregnancy continues, frequency falls, with some increase in amplitude producing noticable ‘Braxton-Hicks’ contractions
=none of these are forceful enough to have any effect on the fetus
What hormones cause a sudden increase in frequency and force of contractions at the onset of labour?
Prostaglandins: enhance the release of calcium from intracellular stores
Oxytocin: peptide hormone which is secreted from the posterior pituitary gland and acts by lowering the threshold for triggering AP’s
Onset of labour is associated with increased prostaglandin synthesis/release in conjunction with increased sensitivity to oxytocin
When is the cervis fully dilated?
At 10cm
What is the ferguson reflex?
As contractions increse, the ferguson reflex increases oxytocin secretion massively
-sensory receptors in cervix and vagina are stimulated by contractions
-excitation passes via afferent nerves to the hypothalamus promoting oxytocin release
=this positive feedback makse contractions more forceful and frequent
What is brachystasis?
At each contraction muscle fibres shorten but do not relax fully
- the uterus, particularly the fundal region shortens progressively
- this pushes the presenting part into the birth canal and stretches the cervix over it
- descent of the presenting part (head usually) occurs progressively during labour until it engages with the pelvis
What will induce labour if given medically?
Prostaglandins
What causes brief reductions in fetal heart rate?
Increasingly forceful uterine contractions may temporarily reduce placental blood flow, so reduce oxygen supply to the fetus
-if reductions in flow are greater than usual, the fetus can become distressed
When does the first stage of labour end?
When cervical dilatation reaches 10cm
How long does the second stage of pregnancy last for?
- up to 1 hr in mulitparous woman
- up to 2 hrs in primigravida (first pregnancies)
What happens in the second stage of labour?
- descended head flexes as it meets pelvis floor, reducing the diameter of presentation
- internal rotation
- sharply flexed head descends to the vulva, stretching the vagina and perineum
- head is then delivered (crowning) and as it emerges it rotates back to its original position and extends
- shoulders then rotate followed by the head and the shoulders deliver followed rapidly by the rest of the fetus (be aware of shoulder distocia-cause brachial plexus injuries)
Second stage of labour ends with delivery of the fetus
What is shoulder distocia?
When the baby’s anterior shoulder gets caught on the mothers pubic bone
- turtle sign: retraction of baby’s head
- can cause clavicle fracture or brachial plexus injury
What happens in the third stage of labour?
-with the fetus removed there is a powerful uterine contraction which separates the placenta positioning it into the upper part of the vagina/lower uterine segment
-the placenta and membranes are then expelled within about 10 mins
=this completes the third stage of labour
(contraction of the uterus also compresses the blood vessels and reduces bleeding)
(it is normally enhanced by administration of an oxytocic drug)
What does the first breath of the fetus cause?
First breath is triggered by multiple stimuli (delivery trauma, temp change etc)
- it causes a dramatic fall in pulmonary vascular resistance, reducing the pulmonary arterial pressure and increasing left atrial pressure relative to the right atrial pressure
- atrial pressure change shuts the foramen ovale and the rising arterial pO2 causes the ductus arteriosus to constrict
- ductus venosus constricts so that all the blood entering the liver passes through the hepatic sinusoids
What scoring system is used to assess fetal wellbeing soon after delivery?
APGAR score -used to asses wellbeing soon after delivery and then 5 mins after -generates a score from 1-10 (higher number= healthier baby) A- appearance (blue) P- pulse G- grimace A- activity R- respiration
What can the elements of labour be classified into?
The powers: delivery of fetus is dependent of contraction of the myometrium which has undergone hypertrophy and hyperplasia in pregnancy, contraction and retraction of multidirectional smooth muscle fibres causes progressive shortening particularly in the fundus
The passage: formed by the bony pelvis and soft tissues, the fetus flexes, extends, and rotates as it passes through the birth canal
The passenger: size and presentation of the fetus is critical, moulding of the fetal cranium may occur since cranial sutures are not yet fused
How can uterine contractions be assessed?
In terms of frequency, amplitude and duration
What can slow labour?
Resistance of the soft tissue
What is the dimension of the pelvic inlet/outlet?
Pelvic inlet: shorter in anterior-posterior plane: 10.5 cm
B/w pelvic inlet and outlet the mid-cavity is circular: 12 cm
Pelvic outlet is narrowest mediolaterally: 11 cm
Why may there be a failure to progress in labour?
- inadequate power (insufficient uterine contractions)
- inadequate passage (abnormal bony pelvis/rigid perineum)
- abnormalities of the passenger
What is progress in labour plotted graphically on?
Partogram
