Labor
District 1 ACOG Medical Student
Education Module 2008
Labor
Labor is the physiologic process by which
a fetus is expelled form the uterus to the
outside world.
It involves the sequential integrated
changes in the uterine decidua, and
myometrium.
Changes in the uterine cervix tend to
precede uterine contractions
Labor - Mechanics
Uterine contractions have two major goals:
To dilate cervix
To push the fetus through the birth canal
Success will depend on the three P’s:
Powers
Passenger
Passage
Power
Uterine contractions
 Power refers to the force generated by the
contraction of the uterine myometrium
 Activity can be assessed by the simple
observation by the mother, palpation of the
fundus, or external tocodynamometry.
 Contraction force can also be measured by direct
measurement of intrauterine pressure using
internal manometry or pressure transducers.
Power
There is no specific criteria for adequate
uterine activity
Generally 3-5 contractions in a 10 minute
period is considered adequate labor
Passenger
Passenger =fetus
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Fetal variables that can affect labor:
Fetal size
Fetal Lie – longitudinal, transverse or oblique
Fetal presentation – vertex, breech, shoulder,
compound (vertex and hand), and funic (umbilical
cord).
Attitude – degree of flexion or extension of the fetal
head
Position
Station – degree of descent of the presenting part of the
fetus, measured in centimeters from the ischial spines
Number of fetuses
Presence of fetal anomalies – hydrocephalus,
sacrococcygeal teratoma
Passage
Passage = Pelvis
 Consists of the bony pelvis and soft tissues of the
birth canal (cervix, pelvic floor musculature)
 Small pelvic outlet can result in cephalopelvic
disproportion
 Bony pelvis can be measured by pelvimetry but it
not accurate and thus has been replaced by a
clinical trial of labor
Passage
 www.uptodate.com
Passage - Pelvimetry

www.uptodate.com
The Stages of Labor
First Stage
 Interval between the onset of labor and full
cervical dilation
 Two phases:
 Latent phase – onset o f labor with slow cervical
dilation to ~4 cm and variable duration
 Active phase – faster rate of cervical change, 11.2 cm /hour, regular uterine contractions
The Labor Curve
 First stage - A: latent phase; B + C + D: active phase; B:
acceleration; C: maximum slope of dilation; D: deceleration; E:
second stage.
Adapted from: Friedman. Labor: Clinical evaluation and
management, 2nd ed, Appleton, New York 1978.
Labor
 Freidman’s
curve is a good
guideline for
expected
progression in
labor and
therefore
helpful to note
abnormal labor
patterns.
Labor
NulliG
MultiG
1st Stage
Active
phase
Duration
6-18 h
2-10 h
Dilation
~1 cm/h
~1.5 cm/h
Arrested
>2 h
>2h
2nd Stage
0.5-3 h
5-30 min
3rd Stage
0-30 min
0-30 min
Labor
Variables associated with longer labors:
Electronic fetal monitoring
Narcotic use
Maternal age >30
Ambulation
Labor – Second Stage
 Interval between full cervical dilation to delivery
of the infant.
 Characterized by descent of the presenting part
through the maternal pelvis and expulsion of the
fetus.
 Indications of second stage:
 Increased maternal show
 Pelvic/rectal pressure
 Mother has active role of pushing to aid in fetal
descent.
Labor – Second Stage
 Examining the fetal head during the second
stage may become difficult due to molding
 Molding is the alteration of the fetal cranial
bones to each other as a result of compressive
forces of the maternal bony pelvis.
 Caput is the localized edematous area on the
fetal scalp caused by pressure on the scalp by the
cervix.
 PrimiG – 0.5-3 h; mulitG 0-30min
Labor – Third Stage
The time from fetal delivery to delivery of
the placenta
Three signs of placental separation:
Lengthening of umbilical cord
Gush of blood
Fundus becomes globular and more anteverted
against abdominal hand
Labor – Third Stage
Placenta is delivered using one hand on
umbilical cord with gentle downward
traction. Other hand on abdomen
supporting the uterine fundus.
Risk factor for aggressive traction is
uterine inversion.
Obstetrical emergency!!
Normal duration between 0-30 min for
both PrimiG and MultiG
Labor – Fourth Stage
 Refers to the time from delivery of the placenta
to 1 hour immediately postpartum
 Blood pressure, uterine blood loss and pulse
rate must be monitor closely ~ 15 minutes
 High risk for postpartum hemorrhage from:
 Uterine atony, retained placental fragments,
unrepaired lacerations of vagina, cervix or
perineum.
 Occult bleeding may occur – vaginal hematoma
 Be suspicious with increased heart rat, pelvic
pain or decreased BP
Cardinal Movements of Labor
 Refers to changes in the fetal head position
during its passage through the canal.
 Seven distinct movements:
Engagement
Descent
Flexion
Internal rotation
Extension
External rotation/restitution
Expulsion
Cardinal Movements of Labor
Engagement
Passage of the widest diameter fetal
presenting part below the plane of the
pelvic inlet
The head is said to be engaged if the
leading edge is at the level of the ishial
spines.
Cardinal Movements of Labor
Descent
Refers to the downward passage of the
presenting part through the bony pelvis
Not steady process
Greatest at deceleration phase of first
stage and during 2nd stage of labor
Cardinal Movements of Labor
Flexion
Occurs passively as the head descends
due to the shape of the bony pelvis.
Partial flexion occurs naturally but
complete flexion usually occurs only in
the labor process
Complete flexion places the fetal head in
optimal smallest diameter to fit through
the pelvis
Cardinal Movements of Labor
Internal Rotation
Rotation of the fetal head from occiput
transverse to occiput either in anterior or
posterior position
Occurs passively due to the shape of the
bony pelvis
Cardinal Movements of Labor
Extension
Occurs when the fetus has descended to
the level of the vaginal introitus
When occiput is just past the level of the
symphysis, the angle of the birth canal
changes to upward position
Cardinal Movements of Labor
External Rotation/Restitution
As the head is delivered, it rotates back to
its original position prior to internal
rotation
It aligns anatomically with the fetal torso
The release of the passive forces on the
fetal head allows it to return to
appropriate position
Cardinal Movements of Labor
Expulsion
Delivery of the fetus
After delivery of the fetal head, descent
and intraabdominal pressure by mother
brings shoulder to the level of the
symphysis
Downward traction allows release of the
shoulder and the fetus is delivered.
Cardinal Movements of Labor
In Summary
Know the different stages of labor
Know the labor curve
Know the cardinal movements of labor
Know the causes of postpartum
hemorrhage
The remaining talk regarding labor,
induction, augmentation, surveillance and
complications will be discussed in
following lectures…