– not related to progesterone changes!!! Probably not related to sudden changes in estrogen. Not related to sharp surges in oxytocin. Not related to prostaglandin changes.
multifactorial, not fully understood – fetal signal? Size of uterus? CNS activity?
prodromal phase (Labor stage 0) -
Ý frequency of contractions, pelvic and back pressure
Cervical Ripening
–ground substance altered; Ý water; effacement and gradual dilation; cervix axis shifts and moves anterior
mid 3rd trimester–almost no contractions; 1 wk prelabor–contraction freq. sharply
Ý (also pelvic/back pressure)Þ labor
Changes at Term
–cervical ripening, formation of gap junctions and oxytocin receptors, Ý corticotropin releasing hormone
during ripening, collagen breaks down and water-retaining hyaluronic acid increases
there is probably a physiologic trigger: PGs?, Relaxin?
Role of Oxytocin
– nanopeptide released by posterior pituitary – not the trigger for labor; may sustain labor
important in 3rd stage and involution of uterus; promotes mRNA production for collagenases, interleukin-8, other proteins
stimulates let-down by mammary duct myoepithelial cells to
Ý lactation (doesn’t make or store milk, just Ý release)
Prostaglandins
– synthesized by decidua. Common types: PGF2a , E2, PGFM
concentrated in forewaters (after head enters pelvis there are different chemical environments in front of and behind face)
most likely do not initiate normal term labor; enhancement possible (
Ý oxytocin receptors)
Uterine Contractions
– lower segment contracts weakly and thins out, Ý in size over time; upper segment contracts
downward onto descending fetus from each Cornu – top of uterus toward the midline and
ß in size
painful as cervix dilates and cervical and uterine ganglia are compressed by myometrium
Ý
by cervical stretching ("Ferguson Reflex") and "stripping" (membrane separates from decidua; Ý local PGF2a release)
pressure 40-60 mmHg, ~1 min long w at least 1-2 minutes between – powerful enough to compress blood vessels in uterus
contractions of 200-250 MVU/10 minutes adequate for progress (Montevedeu Units, named after inventor)
Forces of Labor
– five P’s
Pelvis (passage) – diagonal conjugate normal, side walls parallel, nonprominent spines, curved sacrum, wide suprapubic angle, fetal enlargement, descent with pressure. No obstructions.
Passenger – too large (>9 lbs)
Þ dystocia; too small or abnormal muscle toneÞ malpresentation; head position affects diameters – occiput (top of the head) has to be anterior (anterior=mom’s symphysis, posterior=mom’s sacrum)
Power (oxytocics, prior deliveries)–prior deliveries
Ý myometrial gap junctions, so uterus does better job 2nd time around
Pain/Pain medications – if it hurts, mom won’t contract well
(Physician) Mechanical interventions – fingers, knives and forceps and vaccum and pull on ankles and feet
Cardinal Mechanisms of Labor
– baby starts out floating then must change axis (straighten out; head down 97% of time)
Engagement
(Descent); Flexion (baby bounces and flexes head to try to fit through birth canal); Internal rotation; Descent; Extension (of head as hits perineum); External Rotation (Restitution)
Impending Labor
–Ý freq. of uterine tonic activity;Ý liquidity of cervical mucus with passage of copious mucus(mucus plug)
blood-tinged mucus: "Bloody show" – physician doesn’t have to be notified yet
Stages of Labor
– three stages:
(1) First Stage (Cervica and Dilatation)
– 2 phases, latent (closed
Þ 3-4 cm) and active (3 cmÞ 10 cm)
Latent Phase
: contractions
Ý intensity and frequency, culminating in strong, forceful, regular contractions and cervical changes
Active Phase
: cervix achieves complete effacement and dilatation over brief and relatively predictable interval of time
(2) Second Stage (Expulsive)
– 2 hrs without, 3 hrs with epidural; 1-2cm/hr – cervix has reached 10 cm dilatation and the presenting part descends through the maternal pelvies to the vaginal introitus. Delivery of infant then occurs.
from complete dilatation until the feet cross the perineum is the second stage: fetal expulsion
(3) Third Stage (Separation and Expulsion of Placenta)
– from time of delivery to the delivery of placenta– » 30min
some authors would like to extend this stage to include immediate postpartum period or to assign the "fourth stage" to the immediate puerperium
Friedman Curve
– in ’78, Emmaual Friedman analyzed progress of nulliparous and multiparous patients through labor based on cervical dilatation and descent. The Friedman Curve is +2 SD from mean, (not average – above the mean)
deviation from curve was associated with more frequent adverse outcomes: sepsis, asphyxia, cesarean, deaths
Typical Course of Labor and Delivery
present with frequent, uncomfortable contractions; general maternal and fetal condition assessed; confirmation of membrane rupture if appropriate; assessment of fetal heart rate; vaginal examination
patients ambulates, showers, sits and lies down for comfort as contraction increases
serial exams are perfoed once active phase is ided to confirm progress; palpation of contraction intensity
monitors and analgesia/anesthesia may be used for specific risks or situations, or pitocin to
Ý contraction intensity
patient instructed to push when 10cm (speeds descent); fetal head appears at perineum (distends to permit delivery); umbilical cord loops reduced, nares and mouth suctioned; forceps, vacuum, episiotomy may be used in some cases
shoulders appear, rotate to AP and are allowed to deliver; torso then follows rapidly
cord is clamped and divided, the infant is placed on maternal chest/abdomen; cord blood samples obtained (check pH)
APGARs are assigned at 1 and 5 mins; baby receives ophthalmic prophylaxis and Vitamin K
placenta delivers; pitocin is administered or uterine massage is performed as needed
maternal soft tissues are inspected/repaired as needed; blood loss is assessed