NUR1 233 Chapter Notes - Chapter 15: Shoulder Dystocia, Perineum, Levator Ani

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Published on 13 Jun 2012
School
McGill University
Department
Nursing
Course
NUR1 233
Chapter 15: Labor and Birth Processes
Factors affecting labor
Passenger: fetus and placenta
Passageway: birth canal
Powers: contractions
Position of mother
Psychologic response
Passenger
Size of fetal head, fetal presentation, fetal lie, fetal attitude, fetal position
Placenta: passenger too rarely impedes process of labor in normal vaginal birth exception is placental previa
Size of fetal head
o Fetal skull: two parietal bones, two temporal boens, frontal bone and occipital bone
o Sutures: sagittal, lambodial, coronal, and frontal membranous unite fetal skull bones
o Fontanels: membrane-filled spaces located where sutures intersect
Anterior: larger diamond shaped 3cm x 2cm at junction of sagittal, coronal, and frontal sutures
closes by 18 months after birth
Posterior: smaller triangular shaped 1cm x 2cm at junction of sutures of two parietal bones and
occipital bone - closes by 6-8 weeks after birth
o Sutures and fontanels make skull flexible to accommodate the infant brain, which continues to grow for some
time after birth
o Molding: bones not firmly united slight overlapping of the bones or molding of shape of head after labor
Provides adaptation to the various dimeters of the maternal pelvis
Head assumes normal shape within 3 days after birth
o During labor, after rupture of membranes, palpation of fontanels and sutures during vaginal examination
reveals fetal presentation, position, and attitude
o Shoulder: diameter smaller than skull, facilitating passage through the birth canal
o Hips: circumference small enough as well
Fetal presentation
o Part of the fetus that enters the pelvic inlet first and leads through the birth canal during labor at term
o 3 main presentations
Cephalic: head first 96% - usually occiput
When presenting part is occiput, the presentation is noted as vertex
Breech: buttocks or feet first 3% - usually sacrum
Shoulder: 1% - usually scapula
o Factors that determine the presenting part: fetal lie, fetal attitude, extension or flexion of head
Fetal lie
o Relation of the long axis (spine) of fetus to the long axis (spine) of mother
o Longitudinal lie (vertical): long axis of the fetus is parallel with the long axis of the mother
Either cephalic or breech, depending on fetal strucuture that first enters the mother’s pelvis
o Transverse lie (horizontal or oblique): long axis of the fetus is at the right angle diagonal to the long axis of
the mother
Has to be C-section
Oblique lie: long axis of the fetus is lying at an angle to the long axis of the mother less common
and usually converts to a longitudinal or transverse lie during labor
Fetal attitude
o Relation of the fetal body parts to one another
o Fetus assumes a characteristic posture (attitude) in utero partly because of the mode of fetal growth and partly
because of the way the fetus conforms to the shape of the uterine cavity
o General flexion: back of fetus is rounded so that chin flexed on chest, thighs flexed on abdomen, and legs
flexed at knees arms crossed over thorax, and umbilical cord between arms and legs
Deviations from this may cause difficulties during childbirth
o Cephalic presentation: fetal head may be extended or flexed in a manner that presents a head diameter that
exceeds the limits of the maternal pelvis, leading to prolonged labor, forceps or vacuum-assisted birth, or
cesarean
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o Certain crtical diameters of head measured
Biparietal diameter: 9.25cm at term is largest transverse diameter and an important indicator of fetal
head size
In a well-flexed cephalic presentation the bipariental diameter is the widest part of the head
entering the pelvic inlet
Anteroposterior diameters: many of them
Suboccipitobregmatic diameter: smallest and most critical 9.5cm at term when head in
complete flexion, this diameter allows fetal head to pass through true pelvis easily
As head more extended, anteroposterior diameter widens and the head may not be able to
enter the true pelvis
Fetal position
o Position is the relation of the presenting part (occiput (lower back of head), sacrum, mentum (chin), sinciput
(front of head), vertex (back of head)), to the four quadrants of the mother’s pelvis
o Denoted by three-letter abbreviation
o First letter: denotes the location of the presenting part in the right (R) or left (L) side of the pelvis
o Middle letter: specific presenting part of the fetus (O for occiput, S for sacrum, M for mentum, and Sc for
scapula)
o Third letter: location of the presenting part in relation to the anterior (A), posterior (P) or transverse (T)
portion of the maternal pelvis
o Ex. ROA: occuput is the presenting part and is located in the right anterior quadrant of the maternal pelvis
o Station: relation of the presenting part of the fetus to an imaginary line drawn between the maternal ischial
spines and is a measure of the degree of descent of the presenting part of the fetus through the birth canal
Placement of the presenting part is measured in cm above or below the ischial spines
Lowermost portion of the presenting part is 1cm above spines: station is -1
Presenting part at the level of the spines: station is 0
Presenting part 1cm below the spines: station is +1
Birth is imminent when the presenting part is at +4/+5
Station of the presenting part should be determined when labor begins so that the rate of descent of
the fetus during labor can be accurately determined
o Engagement: used to indicate the largest transverse diameter of the presenting part (usually the biparietal
diameter) has passed through teh maternal pelvic brim or inlet into the true pelvis and usually corresponds to
station 0
Occurs in the weeks just before labor begins in nulliparas and may not occur before or during labor in
multiparas
Can be determined by abdominal or vaginal examination
Passageway
Composed of mother’s rigid bony pelvis and the soft tissues of the cervix, pelvis floor, vagina, and introitus (external
opening to the vagina)
Maternal pelvis: plays greatest role in labor process because the fetus must successfully accommodate itself to the
relatively rigid passageway
o Its size and shape must be determined before labor begins
Bony pelvis
o Formed by fusion of the ilium, ischium, and sacral bones
o Four pelvic joints: symphysis pubis, right and left sacroiliac joints, and the sacrococcygeal joint
o Separated by brim, or inlet, into two parts
False pelvis: part above the brim and plays no part in childbrearing
True pelvis: part involved in birth, divided into 3 planes inlet or brim, midpelvis or cavity, and
outlet
Pelvic inlet: upper border of the true pelvis formed anteriorly by upper margins of the
pubic bone, laterally by the iliopectineal lines along the innominate bones, and posteriorly by
anterior, upper margin of the sacrum and sacral promontory
Pelvic cavity: curved passage with a short anterior wall and a longer concave posterior wall
bounded by the posterior aspect of the symphysis pubis, the ischium, a portion of the ilium,
the sacrum, and the coccyx
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Pelvic outlet: lower border of the true pelvis it is ovoid, somewhat diamond shaped and
bounded by the pubic arch anteriorly, the ischial tuberosities laterally, and the top of the
coccyx posteriorly
The latter part of pregnancy, the coccyx is movable
o Pelvic canal varies in size and shape at various levels
o The diameters at the plane of the pelvic inlet, midpelvis, and outlet, plus the axis of the birth canal determine
whether vaginal birth is possible and the manner by which the fetus may pass down the birth canal
o Subpubic angle: determines the type of pubic arch
o The fetus must first pass beneath the pubic arch, a narrow subpubic angle is less accommodating than a
rounded wide arch
o 4 basic types of pelves are classified as follows:
Gynecoid: classical female type most common, with major gynecoid pelvic features in 50% of all
women
Android: resembling male pelvis less common
Anthropoid: resembling the pelvis of anthropoid apes less common
Platypelloid: flat pelvis least common
Mixed types of pelves are more common than are pure types
o Assessment of bony pelvis can be performed during the first prenatal evaluation and need not be repeated if
the pelvis is of adequate size and suitable shape
o 3rd trimester: examination may be more thorough and results more accurate because there is relaxation and
increased mobility of the pelvic joints and ligaments as a result of hormonal influences
Widening of the joint of the symphysis pubis an dthe resulting instability may cause pain in any or all
of the pelvic joints
o Examiner doesn’t have direct access to the bony structures and because bones are covered with varying
amounts of soft tissue, estimates of size and shape are approximate
o Precise bony pelvis measurements can be determined by use of computed tomography, ultrasound, or x-ray
firls
Soft tissues
o Distensible lower uterine segment, cervix, pelvic floor muscles, vagina, and introitus
o Before labor uterine composed of: uterine body (corpus) and cervix (neck)
o After labor has begun: uterine contractions cause uterine body to have a thick and muscular upper segment
and a thin-walled, passive, muscular lower segment
Physiological retraction ring separated two segments
Lower uterine segment: gradually distends to accomodate the intrauterine contents as the wall of the
upper segment thickens and its accommodating capacity is reduced
o Contractions of the uterine body thus exert downard pressure on the fetus, pushing it against the cervix
o Cervix: effaces (thins) and dilates (opens) sufficiently to allow first fetal portion to descend into vagina
o Fetus descents: cervix drawn upward and over this first portion
o Pelvic floor: muscular layer that separates the pelvic cavity above from the perineal space below
This structure helps fetus rotate anteriorly as it passes through teh birth canal
o Soft tissues of vagina develop throughout pregnancy until at term the vagina can dilate to accommodate the
fetus and permit passage of the fetus to the external world
Powers
Primary powers: involuntary uterine contractions signal the beginning of labor
o Originate at certain pacemaker points in the thickened muscle layers of the upper uteirne segment
o Contractions then move downward over the uterus in waves, separated by short rest periods
o Frequency: time from beginning of one contraction to the beginning of the next
o Duration: length of contraction
o Intensity: strength of contraction
o Responsible for effacement and dilation of the cervix and descent of the fetus
o Effacement: shortening and thinning of the cervix during the first stage of labor
Cervix: normally 2-3cm long and 1cm thick, obliterated or “taken up” by a shortening of the uterine
muscle bundles during the thinning of the lower uterine segment that occurs in advancing labor
Only a thin edge of the cervix can be palpated when the effacement is completed
Effacement generally advanced in first time term pregnancy before more than slight dilation occurs
In subsequent pregnancies, effacement and dilation of the cervix tend to progress together
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Document Summary

Size of fetal head, fetal presentation, fetal lie, fetal attitude, fetal position. Placenta: passenger too rarely impedes process of labor in normal vaginal birth exception is placental previa. Size of fetal head: fetal skull: two parietal bones, two temporal boens, frontal bone and occipital bone, sutures: sagittal, lambodial, coronal, and frontal membranous unite fetal skull bones, fontanels: membrane-filled spaces located where sutures intersect. Anterior: larger diamond shaped 3cm x 2cm at junction of sagittal, coronal, and frontal sutures. Provides adaptation to the various dimeters of the maternal pelvis. Fetal presentation: part of the fetus that enters the pelvic inlet first and leads through the birth canal during labor at term, 3 main presentations. Cephalic: head first 96% - usually occiput. When presenting part is occiput, the presentation is noted as vertex. Breech: buttocks or feet first 3% - usually sacrum. Shoulder: 1% - usually scapula: factors that determine the presenting part: fetal lie, fetal attitude, extension or flexion of head.

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