Transcript Fetal Presentation

Dr. Areefa Al Bahri
Ch. 5
The Birth Experience
Physiological, psychological, and emotional
changes that take place during pregnancy help to prepare
the woman for labor and birth. Near the end of the pregnancy,
the fetus continues to develop physiological abilities
that facilitate successful adaptation for the transition
from in utero life to the outside environment.
The Process of Labor and Birth
A number of forces affect the progress of labor and help to
bring about childbirth. These critical factors are often
referred to as the “P’s” of labor:
1. Powers (physiological forces)
2. Passageway (maternal pelvis)
3. Passenger (fetus and placenta)
4. Passageway Passenger and their relationship
(engagement, attitude, position)
5. Psychosocial influences (previous experiences, emotional
status)
POWERS
The powers are the physiological forces of labor and birth that
include the uterine contractions and the maternal pushing
efforts. The uterine muscular contractions, primarily responsible
for causing cervical effacement and dilation, also move the fetus
down toward the birth canal during the fi rst stage of labor.
Uterine contractions are considered the primary force of labor.
Once the cervix is fully dilated, the maternal pushing efforts
serve as an additional force. During the second stage of labor,
use of the maternal abdominal muscles for pushing (the
secondary force of labor) adds to the primary force to facilitate
childbirth.
Characteristics of Uterine Contractions
Contractions are a rhythmic tightening of the uterus that occurs
intermittently. Over time, this action shortens the individual uterine muscle
fi bers and aids in the process of cervical effacement and dilation, birth, and
postpartal involution (the reduction in uterine size after birth). Each
contraction consists of three distinct components: the increment (building of
the contraction), the acme (peak of the contraction) and the decrement
(decrease in the contraction). Between contractions, the uterus normally
returns to a state of complete relaxation. This rest period allows the uterine
muscles to relax and provides the woman with a short recovery period that
helps her to avoid exhaustion. In addition, uterine relaxation between
contractions is important for fetal oxygenation as it allows for blood fl ow
from the uterus to the placenta to be restored.
The lower uterine segment becomes thin-walled and passive.
The boundary between the upper lower uterine segments
becomes marked by a ridge on the inner uterine surface, known
as the “physiological retraction ring.” With each contraction,
the uterus elongates. Elongation causes a straightening of the
fetal body so that the upper body is pressed against the fundus
and the lower, presenting part is pushed toward the lower
uterine segment and the cervix. The pressure exerted by the
fetus is called the fetal axis pressure. As the uterus elongates,
the longitudinal muscle fibers are stretched upward over the
presenting part. This force, along with the hydrostatic pressure
of the fetal membranes, causes the cervix to dilate (open).
The coordinated efforts of the contractions help to bring about
effacement and dilatation of the cervix. Effacement is the process of
shortening and thinning of the cervix. As contractions occur, the
cervix becomes progressively shorter until the cervical canal
eventually disappears. The amount of cervical effacement is usually
expressed as a percentage related to the length of the cervical canal,
as compared to a non effaced cervix.
For example, if a cervix has thinned to half the normal length of a
cervix it is considered to be 50% effaced. Dilation is the opening and
enlargement of the cervix that progressively occurs throughout the
first stage of labor. Cervical dilation is expressed in centimeters and
full dilation is approximately 10 cm. With continued uterine
contractions, the cervix eventually opens large enough to allow the
fetal head to come through. At this point, the cervix is considered
fully dilated or completely dilated and measures 10 cm.
Maternal Pushing Efforts
After the cervix has become fully dilated, the laboring
woman usually experiences an involuntary “bearing down”
sensation that assists with the expulsion of the fetus. At
this time, the woman can use her abdominal muscles to aid
in the expulsion. It is important to remember that the cervix
must be fully dilated before the patient is encouraged
to push. Bearing down on a partially dilated cervix can
cause cervical edema and damage and adversely affect the
progress of the labor. For most women, the urge to bear
down generally occurs when the fetal head reaches the
pelvic fl oor. Women who have a strong urge to push often
do so more effectively than women who force themselves
to push without experiencing any sensations of pressure.
PASSAGEWAY
The passageway consists of the maternal pelvis and the soft
tissues. The bony pelvis through which the fetus must pass is
divided into three sections: the inlet, midpelvis (pelvic cavity),
and outlet. Each of these pelvic components has a
unique shape and dimension through which the fetus must
maneuver to be born vaginally. In human females, the four
classic types of pelvis are the gynecoid, android, platypelloid,
and anthropoid.
PASSENGER
The passenger is referred to as the fetus and the fetal
membranes. In the majority (96%) of pregnancies, the fetus
presents in a head-fi rst position. The fetal skull, usually the
largest body structure, is also the least flexible part of the
fetus. However, because of the sutures and fontanels, there is
some flexibility in the fetal skull. These structures allow the
cranial bones the capability of movement and they overlap in
response to the powers of labor. The overlapping or
overriding of the cranial bones is called molding.
The fetal skull, or cranium, consists of three major components:
the face, the base of the skull, and the vault of the cranium (roof).
The facial bones and the cranial base are fused and fixed. The
cranial base is made up of two temporal bones. The cranial vault
is composed of five bones: two frontal bones, two parietal bones,
and the occipital bone. These bones, which are not fused, meet at
the sutures. The sutures of the fetal skull are composed of strong
but flexible connective tissue that fills the spaces that lie between
the cranial bones.
The sagittal suture lies between the parietal bones and runs in an anteroposterior
direction between the fontanels, dividing the head into a right and a left side. The
lambdoidal suture extends from the posterior fontanel and separates the occipital
bones from the parietal bones. The coronal sutures are located between the
frontal and parietal bones. They extend from the anterior fontanel laterally and
separate the parietal from the frontal bones. The frontal (mitotic) suture lies
between the frontal bones and extends from the anterior fontanel to the
prominence between the eyebrows. Two membrane-fi lled spaces are present
where the suture lines meet. These spaces are referred to as the anterior and
posterior fontanels. The anterior fontanel is the larger of the two and measures
approximately 0.8 1.2 inch (2 3 cm). It is diamond shaped and is positioned
where the sagittal, frontal, and coronal sutures
intersect. The anterior fontanel remains open until approximately 18 months of
age to allow normal brain growth to occur. The posterior fontanel is triangular in
shape and is much smaller than the anterior fontanel. It measures approximately
0.8 inch (2 cm) at its widest point. The posterior fontanel is positioned where the
lambdoidal and sagittal sutures meet. Shaped like a small
triangle, it closes at approximately 6 to 8 weeks after birth.
Fetal Presentation
The fetal presentation refers to the fetal part that enters the pelvic inlet first and
leads through the birth canal during labor. The fetal presentation may be cephalic,
breech, or shoulder. The part of the fetal body first felt by the examining finger
during a vaginal examination is the “presenting part.” The presenting part is
determined by the fetal lie and attitude.
CEPHALIC PRESENTATION
fetal head will be first to come into contact with the maternal cervix. Cephalic
presentations occur in approximately 95% of pregnancies. There are four types of
cephalic presentations
Vertex. The fetal head presents fully flexed. This is the
most frequent and optimal presentation as it allows the smallest
suboccipitalbregmatic diameter to present. It is called a “vertex presentation.”
Military. In the military position, the fetal head presents
in a neutral position, which is neither flexed nor extended. The occipitofrontal
diameter presents to the maternal pelvis and the top of the head is the presenting
part.
Brow. In the brow position, the fetal head is partly
extended. This is an unstable presentation that converts toFace.
Face presentation.
the fetal head is fully extended. The submentobregmatic diameter presents to the
maternal pelvis and the face is the presenting part
The following advantages are associated with a cephalic
presentation:
• The fetal head is usually the largest part of the infant.
Once the fetal head is born, the rest of the body usually delivers
without complications.
• The fetal head is capable of molding. There is sufficient time
during labor and descent for molding of the fetal head to occur.
Molding helps the fetus to maneuver through the maternal birth
passage.
• The fetal head is smooth and round, which is the optimal shape to
apply pressure to the cervix and aid in dilation.
Other presentations (e.g., breech, shoulder) are associated with
difficult, prolonged labor and often require cesarean births. They are
called malpresentations.
BREECH PRESENTATION
A breech presentation occurs when the fetal buttocks enter the maternal pelvis first.
Breech presentations occur in approximately 3% of births and are classified
according to the attitude of the fetal
hips and knees. Breech presentations are more likely to occur in preterm births or in
the presence of a fetal abnormality such as hydrocephaly (head enlargement due to
fluid) that prevents the head from entering the pelvis. They are also associated with
abnormalities of the maternal uterus or pelvis. Since many factors can compromise
the normal labor and birth process associated with breech presentations, delivery is
usually accomplished via cesarean section. There are three types of breech
presentations
Frank. The frank breech is the most common of all
breech presentations
Complete (Full). The complete, or full, breech position is
the same as the flexed position with the fetal buttocks
presenting first. The legs are typically flexed.
Footling. In the footling breech position, one or both of
the fetal leg(s) are extended with one foot (“single footling”) or both feet (“double
footling”) presenting first into the maternal pelvis.
Several disadvantages are associated with a breech presentation:
1. An increased risk for umbilical cord prolapsed because the
presenting part may not be covering the cervix (i.e., footling
breech).
2. The presenting part (buttocks, feet) is not as smooth and hard as the
fetal head and is less effective in dilating the cervix.
3. Once the fetal body (abdomen) is delivered, the umbilical cord can
become compressed.
Rapid delivery may be difficult since the fetal head is usually the largest
body part and in this situation, there is no time to allow for molding. In
response to adverse outcomes that have been associated with vaginal
breech births, the American College of Obstetricians and Gynecologists
(ACOG, 2006) has published a Committee Opinion concerning planned
breech deliveries.
SHOULDER PRESENTATION
The shoulder presentation is a transverse lie (Fig.
below). This presentation is rare and occurs in fewer
than 1% of births. When a transverse lie is present,
the maternal abdomen appears large from side to
side, rather than up and down. In addition, the
woman may demonstrate a lower than expected (for
the gestational age) fundal height measurement.
Although the shoulder is usually the presenting part,
the fetal arm back, abdomen, or side may present in a
transverse lie. This presentation occurs most often
with preterm birth, high parity, prematurely ruptured
membranes, hydramnios, and placenta previa. It is
important for the nurse to promptly identify a
transverse lie or shoulder presentation since the
infant will almost always require a cesarean birth.
Station
Station refers to the level of the presenting part in relation to the maternal ischial
spines. In the normal female pelvis, the ischial spines represent the narrowest
diameter through which the fetus must pass. The ischial spines is a landmark to
identify station zero. To visualize the location of station zero, an imaginary line
may be drawn between the ischial spines.
Engagement has occurred when the presenting part is at station zero. When the
presenting part lies above the maternal ischial spines, it is at a minus station.
Therefore, a station of minus 5 (–5) cm indicates that the presenting part is at the
pelvic inlet. Positive numbers indicate that the presenting part has descended past
the ischial spines.
During labor, the presenting part should continue to descend into the pelvis,
indicating labor progress. As labor advances and the presenting part descends, the
station should also progress to a numerically higher positive station. If the station
does not change in the presence of strong, regular contractions, this finding may
indicate a problem with the relationship between the maternal pelvis and the fetus
(“cephalopelvic disproportion”).
Position
Position refers to the location of a fixed reference point on the fetal
presenting part in relation to a specific quadrant of the maternal
pelvis (Fig. 12-10). The presenting part can be right anterior, left
anterior, right posterior, and left posterior. These four quadrants
designate whether the presenting part is directed toward the front,
back, right, or left of the passageway.
Passageway (passenger)
The passageway and the passenger have been identified as two of the
factors that affect labor. The next “P” is the relationship between the
passageway (maternal pelvis) and the passenger (fetus and
membranes). The nurse assesses the relationship between the two
when determining the engagement, station, and fetal position.
Engagement
Engagement is said to have occurred when the widest diameter of the
fetal presenting part has passed through the pelvic inlet. In a cephalic
presentation, the largest diameter is the biparietal; in breech
presentations, it is the intertrochanteric diameter. Engagement can be
determined by external palpation or by vaginal examination.
In primigravidas, engagement usually occurs
PSYCHOSOCIAL INFLUENCES
The first four P’s discussed address the physical forces of labor. The last
“P” (psychosocial influences) acknowledges the many other critical
factors that have an effect on parents such as their readiness for labor
and birth, level of educational preparedness, previous experience with
labor and birth, emotional readiness, cultural influences, and ethnicity.
Transition into the maternal role, and most likely, into the paternal role
as well, is facilitated by a positive childbirth experience. A number of
internal and external influences can affect the woman’s psychological
well-being during labor and birth.
Culturally oriented views of childbirth help to shape the woman’s expectations
and ongoing perceptions of the birth experience. The nurse’s understanding of
the cultural values and expectations attached to childbirth provide a meaningful
framework upon which to plan and deliver sensitive, appropriate care. Cultural
considerations for the laboring woman encompass many elements of the birth
experience including choice of a birth support person strategies for coping with
contractions, pain expression and relief and food preferences.
Signs and Symptoms of Impending Labor
Before the onset of labor, a number of physiological changes occur that signal
the readiness for labor and birth. These changes are usually noted by the
primigravid woman at about 38 weeks of gestation. In multigravidas, they may
not take place until labor begins. It is important for nurses to empower pregnant
women and their families by teaching them about the signs and symptoms of
impending labor. Providing guidelines about when to contact the health care
provider or come to the birth facility helps to demystify the sometimes
confusing events that surround birth and lessen the anxieties that can
accompany the onset of labor.
LIGHTENING
At about 38 weeks in the primigravid pregnancy, the presenting part
(usually the fetal head) settles downward into the pelvic cavity, causing
the uterus to move downward as well. This process, called lightening,
marks the beginning of engagement. This downward settling of the
uterus may decrease the upward pressure on the diaphragm and result
in easier breathing. The downward settling may also lead to the
following maternal symptoms:
• Leg cramps or pains
• Increased pelvic pressure
• Increased urinary frequency
• Increased venous stasis, causing edema in the lower
extremities
• Increased vaginal secretions, due to congestion in the
vaginal mucosa
BRAXTON-HICKS CONTRACTIONS
As the pregnancy approaches term, most women become more aware of
irregular contractions called Braxton-Hicks contractions. As the contractions
increase in frequency (they may occur as often as every 10 to 20 minutes),
they may be associated with increased discomfort. Braxton-Hicks
contractions are usually felt in the abdomen or groin region and patients may
mistake them for true labor. It is believed that these contractions contribute to
the preparation of the cervix and uterus for the advent of true labor. BraxtonHicks contractions do not lead to dilation or effacement of the cervix, and
thus are often termed “false labor.”
CERVICAL CHANGES
In the non pregnant woman, the cervix is normally rigid. In preparation for
passage of the fetus, the cervix undergoes many physiological changes. The
cervix softens (“cervical ripening”), stretches, and thins, and eventually is
taken up into the lower segment of the uterus. This softening and thinning is
called cervical effacement.
BLOODY SHOW
During pregnancy the cervix is plugged with mucus. The mucus
plug acts as a protective barrier for the uterus and its contents
throughout the pregnancy. As the cervix begins to soften, stretch,
and thin through effacement, there may be rupture of the small
cervical capillaries. The added pressure created by engagement of
the presenting part may lead to the expulsion of a blood mucus
plug, called bloody show. Its presence often indicates that labor
will begin within 24 to 48 hours. Late in pregnancy, vaginal
examination that involves cervical manipulation may also produce
a bloody discharge that can be confused with bloody show.
Rupture Of The Membranes
About 12% of pregnant women experience spontaneous rupture of the amniotic
sac (“ruptured membranes” or “ruptured bag of waters”) prior to the onset of
labor. In the majority of pregnancies, the amniotic membranes rupture once
labor is well established, either spontaneously or by amniotomy, the artificial
rupture of the membranes by the primary care provider.
Rupture of the membranes is a critical event in pregnancy. If the membranes do
rupture at home, the woman should be taught to immediately contact the
birthing center who will advise her to report for an examination.
It is important for the woman to note the color, amount, and odor of the
amniotic fluid. The fluid should be clear and odorless.
A yellow green tinged amniotic fluid may indicate infection or fetal passage of
meconium and this finding always signals the need for further assessment and
fetal heart rate monitoring. Urinary incontinence (frequently associated with
urgency, coughing, and sneezing) is sometimes confused with ruptured
membranes. The presence of amniotic fluid can be confirmed by a Nitrazine
tape test or by a fern test.
First Stage of Labor
This stage begins with the onset of regular uterine contractions and ends with
complete dilation of the cervix. woman may not always recognize when true
labor actually begins. The first stage of labor is most often the longest stage and
its duration can vary considerably among women. The first stage of labor is
divided into three distinct phases: latent, active, and transition. Factors such as
analgesia, maternal and fetal position, the woman’s body size and her level of
physical fitness can also affect the length of labor.
LATENT PHASE
Labor pains are often initially felt as sensations similar to painful menstrual
cramping and are usually accompanied by low back pain. Contractions during
this phase are typically about 5 minutes apart, last 30 to 45 seconds, and are
considered to be mild. During the latent phase cervical effacement and early
dilation (0 to 3 cm) occurs. The latent phase of labor can last as long as 10 to 14
hours as the contractions are mild and cervical changes occur slowly.
ACTIVE PHASE
The active phase of labor is characterized by more active contractions. The
contractions become more frequent (every 3 to 5 minutes), last longer (60
seconds), and are of a moderate to strong intensity. During the active labor
phase, the woman becomes more focused on each contraction and tends to draw
inward in an attempt to cope with the increasing demands of the labor. Cervical
dilation during this phase advances more quickly as the contractions are often
more efficient. While the length of the active phase is variable, nulliparous
women generally progress at an average speed of 1 cm of dilation per hour
and multiparas at 1.5 cm of cervical dilation per hour.
TRANSITION PHASE
The transition phase is the most intense phase of labor. Transition is
characterized by frequent, strong contractions that occur every 2 to 3 minutes
and last 60 to 90 seconds on average. Fortunately, this phase often does not take
long because dilation usually progresses at a pace equal to or faster than active
labor (1 cm/hr for a nullipara and 1.5 cm/hr for a multipara).
Assessment of the Fetus During Labor and Birth
Fetal assessments include the identification of fetal position and
presentation, and the evaluation of the fetal status. Nurses use a variety of
assessment techniques including observation, palpation, and auscultation.
When assessing a woman in labor, the nurse is able to use observation and
interview skills from the moment the woman comes through the door.
Astute observation assists the nurse in assessing the patient’s level of pain,
her coping abilities
Baseline Fetal Heart Rate
The normal baseline fetal heart rate at term is 110 to 160 beats per minute (bpm). There
are two abnormal variations of the baseline: tachycardia (baseline above 160 bpm); and
bradycardia (baseline below 110 bpm).
TACHYCARDIA.
Tachycardia is generally defi ned as a sustained baseline fetal heart rate greater
than 160 beats per minute for a duration of 10 minutes or longer. A number of
conditions are associated with fetal tachycardia:
• Fetal hypoxia: The fetus attempts to compensate for reduced blood flow by
increasing sympathetic stimulation of the central nervous system (CNS).
Maternal fever:
• Maternal medications: Both parasympathetic drugs
(i.e., atropine, scopolamine) and beta-sympathetic drugs (tocolytic drugs used
to halt contractions) can have a stimulant effect and increase the fetal heart rate.
• Infection: uterine infection (amnionitis)
• Fetal anemia: In response to a decrease in hemoglobin, the FHR increases to
compensate and improve tissue metabolism.
• Maternal hyperthyroidism: Thyroid-stimulating hormone (TSH) may cross
the placenta and stimulate the fetal heart rate (Tucker, 2004).
BRADYCARDIA
Bradycardia is defined as baseline FHR of less than 110 to 120 bpm.
Fetal bradycardia may be associated with:
• Late hypoxia: Myocardial activity becomes depressed and lowers
the fetal heart rate.
• Medications: Beta-adrenergic blocking drugs (e.g., propanolol
[Inderal]).
• Maternal hypotension:
• Prolonged umbilical cord compression
• Bradyarrhythmias: With complete heart block, the FHR
baseline is often as low as 70 to 90 bpm.
Variability
Variability of the FHR is manifested by fluctuations in the baseline fetal
heart rate observed on the fetal monitor. The pattern denotes an irregular,
changing FHR rather than a straight line that indicates few changes in the
rate. The variability of the FHR is a result of the interplay between the fetal
sympathetic nervous system, which assists to increase the heart rate and the
parasympathetic nervous system, which acts to decrease the heart rate.
The absence of or undetected variability is considered non-reassuring.
FHR variability is indicative of an adequately oxygenated neurological
pathway in which impulses are transmitted from the fetal brain to the
cardiac conduction system (Fox, Kilpatrick, King, & Parer, 2000).
Conversely, the absence of variability may indicate normal variations such
as fetal sleep (the sleep state should not last longer than 30 minutes), a
response to certain drugs that depress the CNS, such as analgesics
(meperidine [Demerol], tranquilizers (diazepam [Valium]),
ACCELERATIONS
An acceleration is defined as an increase in the FHR of 15 bpm above the
fetal heart baseline that lasts for at least 15 to 30 seconds. Accelerations
are considered a sign of fetal well-being when they accompany fetal
movement. Thus, when a fetus is active in utero, accelerations are
normally present. When contractions are present, accelerations are often
noted as a response to the contraction.
DECELERATIONS
Decelerations are defined as any decrease in FHR below the baseline
FHR. Decelerations are further defined according to their onset and are
characterized as early, variable, and late.
Early Decelerations
Early decelerations are characterized by a deceleration in the FHR that
resembles a mirror image to the contraction. Therefore, the onset of the
deceleration begins near the onset of the contraction, and the FHR
returns to baseline by the end of the contraction. Early decelerations are
usually repetitive and are commonly observed during active labor and
descent of the fetus
Variable Decelerations
Variable decelerations, as the name implies, are decelerations that are
variable in terms of their onset, frequency, duration, and intensity. The
decrease in FHR below the baseline is 15 bpm or more, lasts at least 15
seconds, and returns to the baseline in less than 2 minutes from the time
of onset (NICHD, 1997) (Fig. 12-19). The deceleration is unrelated to the
presence of uterine contractions. Variable decelerations are thought to be
a result of umbilical cord compression. Thus, the degree by which the
cord
is compressed (partially versus completely) can affect the severity of the
deceleration. The American College of Obstetricians and Gynecologists
(ACOG, 2005) classifies variable decelerations as significant when the
FHR falls below 70 bpm and lasts longer than 60 seconds. In addition, the
Society of Obstetricians and Gynaecologists of Canada (SOGC, 2005)
concurs and further identifi es “non-reassuring” or “atypical” variable
decelerations as:
Late Decelerations
This type of deceleration does not resolve until after the contraction
has ended. Late decelerations indicate the presence of uteroplacental
insufficiency, a decline in placental function. a decrease in blood flow
from the uterus to the placenta results in fetal hypoxia and late
decelerations. Late decelerations require prompt attention and
reporting. The longer the late decelerations persist, the more serious
they become. For example, late decelerations in the presence of an
oxytocin infusion may signal a need to immediately
discontinue the oxytocin infusion, especially if uterine
hyperstimulation is suspected. Nursing interventions that should be
implemented immediately include reporting the late decelerations,
changing the maternal position, discontinuing the oxytocin infusion,
increasing the intravenous fl uids, and administering oxygen by mask.
THE CARDINAL
MOVEMENTS
The cardinal movements, or
mechanisms of labor, have
been used to describe how the fetus
(in a vertex presentation)
passes through the birth canal and
the positional
changes required to facilitate birth
(Fig. 12-23). The cardinal
movements are presented in the
order in which they
occur.
Descent
Four forces facilitate descent, which is the progression of the fetal head
into the maternal pelvis: (1) pressure of the amniotic fluid; (2) direct
pressure of the uterine fundus on the fetal breech; (3) contraction of the
maternal abdominal muscles; and (4) extension and straightening of the
fetal body. The fetal head enters the maternal inlet in the occiput transverse
or the oblique position because the pelvic inlet is widest from side to side.
The sagittal suture is equidistant from the maternal symphysis pubis
and sacral promontory. The degree of fetal descent is measured
by stations.
Flexion
Flexion occurs as the fetal head descends and comes into
contact with the soft tissues of the pelvis, the muscles of the
maternal pelvic floor, and the cervix. The resistance
encountered with these structures causes the fetal chin to flex
downward onto the chest. This position allows the smallest
fetal diameters to enter the maternal pelvis.
Internal Rotation
To fit into the maternal pelvic cavity, which is widest in the
anteroposterior diameter, the fetal head must rotate.
Extension
As the fetal head passes under the maternal symphysis pubis, it meets
with resistance from the pelvic floor. The head pivots and extends with
each maternal pushing effort. The head is born in extension as the
occiput slides under the symphysis and the face is directed toward the
rectum. The fetal brow, nose, and chin then emerge. Restitution Internal
rotation causes the fetal shoulders to enter the maternal pelvis in an
oblique position. After the head is delivered in the extended position, it
rotates briefly to the position it occupied when it was engaged in the
inlet. This movement is termed restitution. The 45-degree turn
of the fetal head facilitates realignment with the long axis of the body.
External Rotation
As restitution continues, the shoulders align in the anteroposterior
diameter, causing the head to continue to turn
farther to one side (external rotation). The fetal trunk
moves through the pelvis with the anterior shoulders
descending fi rst.
Expulsion
After external rotation, maternal pushing efforts bring the
anterior shoulder under the symphysis pubis. Lateral fl exion
of the shoulder and head occurs and the anterior, then
posterior, shoulder is born. Once the shoulders are delivered,
the rest of the body quickly follows.