CONTRACTION STRESS TEST (CST)
This test assesses the ability of the fetus to tolerate transitory decreases in intervillous blood flow that occur with uterine contractions. It uses both external FHR and contraction monitor-ing devices and is based on the presence or absence of late decelerations. These are gradual decreases in FHR below the baseline with onset to nadir of ≥30 s. The deceleration onset and end is delayed in relation to contractions. If 3 contractions in 10 min are not spontaneously present, they may be induced with either IV oxytocin infusion or nipple stimulation. This test is rarely performed because of the cost and personnel time required. The most common indica-tion is a BPP of 4 or 6.
• Negative CST requires absence of any late decelerations with contractions. This isreassuring and highly reassuring for fetal well-being. Management is to repeat the CST weekly. Fetal death rate is only 1 per 1,000 in the next week.
• Positive CST is worrisome. This requires the presence of late decelerations associ-ated with at least 50% of contractions. Fifty percent of positive CSTs are false positive
(meaning the fetus is not hypoxemic). They are associated with good FHR variability. The 50% of true positives are associated with poor or absent variability. Management is prompt delivery.
• Contraindications—CST should not be performed whenever contractions would behazardous to the mother or fetus. Examples include previous classical uterine incision, previous myomectomy, placenta previa, incompetent cervix, preterm membrane rup-ture, and preterm labor.
Table 12-2. Contraction Stress Test (CST)
| Negative CST | No late decelerations are seen in the presence of 3 uterine contractions |
| in 10 min | |
| Assessment: reassuring of fetal well-being | |
| Follow-up: repeat CST weekly as needed | |
| Positive CST | Repetitive late decelerations are seen in the presence of 3 uterine |
| contractions in 10 min | |
| Assessment: worrisome, especially if nonreactive non-stress test | |
| Follow-up: prompt delivery | |
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Chapter 12 l Overview of Antepartum Fetal Testing
| Criteria: negative CST | Absent late decels | |||||||
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| with >3 UCs in 10 min | |||||||
| Reassuring. | ||||||||
| Mgmt. | ||||||||
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| Repeat as indicated. | |||||||
| Criteria: positive CST | >50% late decels | |||||||
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| with >3 UCs in 10 min | |||||||
| Ominous. | ||||||||
| Mgmt. | ||||||||
| Deliver promptly. | ||||||||
| Figure I-12-2. Contraction Stress Test | ||||||||
UMBILICAL ARTERY DOPPLER
This test measures the ratio of systolic and diastolic blood flow in the umbilical artery. The umbilical circulation normally has low resistance, so significant diastolic blood flow is expected.
The systolic/diastolic (S/D) ratio normally decreases throughout pregnancy. This test is pre-dictive of poor perinatal outcome only in IUGR fetuses. Nonreassuring findings, which may indicate need for delivery, are absent diastolic flow and reversed diastolic flow.
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Peak systole
Low diastole
Baseline
NORMAL umbilical artery diastolic flow
ABSENT umbilical artery diastolic flow
Baseline
Baseline
REVERSED umbilical artery diastolic flow
Figure I-12-3. Umbilical Artery Doppler Waveform Patterns
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USMLE Step 2 l Obstetrics
With permission, Institute for Advanced Medical Education, www.iame.com
Figure I-12-4. Normal Umbilical Artery Diastolic Flow
With permission, Institute for Advanced Medical Education, www.iame.com
Figure I-12-5. Absent Umbilical Artery Diastolic Flow
With permission, Institute for Advanced Medical Education, www.iame.com
Figure I-12-6. Reversed Umbilical Artery Diastolic Flow
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Fetal Orientation in Utero 13
ANATOMY OF THE BONY PELVIS
The pelvis is constructed of 4 bones: ileum superior-laterally, ischium inferior-laterally, pubis anteriorly, and the sacrum and coccyx posteriorly. It is held together by the following 4 joints: bilateral sacroiliac joints, the symphysis pubis, and the sacrococcygeal joint. The sacrum has5 vertebrae joined together. The anterior superior edge of the first sacral vertebra is called the sacral promontory.
Landmarks. The pelvis is divided by the linea terminalis into the false pelvis above and thetrue pelvis below. The false pelvis is bordered by lumbar vertebrae posteriorly, by the iliac fossa laterally, and by the abdominal wall anteriorly. The true pelvis is a bony canal formed by pos-terior sacrum and coccyx, lateral ischial, and anterior pubis.
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Types of Pelvic Shapes
Gynecoid shape is the classic female pelvis and is found in 50% of women. The inlet is a roundoval with largest diameter transverse. It has straight side walls, well-curved sacrum, and spa-cious subpubic arch with a 90° angle. Assessment: This pelvis is spacious for the fetal head to pass through.
Android shape is the typical male pelvis and is found in 30% of women. The inlet is triangularwith convergent side walls, shallow sacral curve, and narrow subpubic arch. Assessment: This pelvis is restricted at all levels. Arrest of descent in labor is common.
Anthropoid shape resembles that of anthropoid apes and is found in 20% of women. Theinlet is larger anterior-posteriorly with side walls that converge. Subpubic arch is narrow. Assessment: The fetal head engages anterior-posteriorly, often in occiput posterior position,making delivery difficult.
Platypelloid shape is like a flattened gynecoid pelvis. The inlet is an elongated transverse oval.It has straight side walls with deep sacral curve and wide subpubic arch. Assessment: The fetal head engages transversely and delivers occiput transverse position.
ORIENTATION IN UTERO
Lie
Orientation of the long axis of the fetus to the long axis of the uterus. The most common lie is longitudinal. 99% of fetuses at term.
• Longitudinal: fetus and mother are in same vertical axis
• Transverse: fetus at right angle to mother
• Oblique: fetus at 45° angle to mother
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Figure I-13-1. Longitudinal Fetal Lie Figure I-13-2. Transverse Fetal Lie
Presentation
Portion of the fetus overlying the pelvic inlet. The most common presentation is cephalic. This is 96% of fetuses at term.
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• Cephalic: head presents first
• Breech: feet or buttocks present first. The major risk of vaginal breech delivery isentrapment of the after-coming head.
– Frank breech means thighs are flexed and legs extended. This is the only kind of breech that potentially could be safely delivered vaginally.
– Complete breech means thighs and legs flexed.
– Footling breech means thighs and legs extended.
• Compound: more than one anatomic part is presenting (e.g., head and upper extremity)
• Shoulder: presents first
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Chapter 13 l Fetal Orientation in Utero
OB Triad
Breech Presentations
• Frank: thighs flexed, legs extended
• Complete: thighs and legs flexed
• Footling: thighs and knees extended
Figure I-13-3. Frank Breech Figure I-13-4. Complete Breech
Figure I-13-5. Footling Breech
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Position
Relationship of a definite presenting fetal part to the maternal bony pelvis. It is expressed in terms stating whether the orientation part is anterior or posterior, left or right. The most com-mon position at delivery is occiput anterior.
• Occiput: with a flexed head (cephalic presentation)
• Sacrum: with a breech presentation
• Mentum (chin): with an extended head (face presentation)
Attitude
Degree of extension-flexion of the fetal head with cephalic presentation. The most common attitude is vertex.
• Vertex: head is maximally flexed
• Military: head is partially flexed
• Brow: head is partially extended
• Face: head is maximally extended
Station
Degree of descent of the presenting part through the birth canal; expressed in centimeters above or below the maternal ischial spine.
Landmarks
| Occipital bone | Lambdoid | |
| suture | ||
| Posterior | ||
| fontanelle | ||
| Parietal | ||
| Sagittal | bones | |
| suture | ||
| Anterior | ||
| fontanelle | Coronal | |
| Frontal bones | suture | |
| (metopic) | ||
| suture |
Positions
OA
ROA LOA
| ROT | LOT | ||
ROP LOP
OP
Figure I-13-6. Landmarks and Positions
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Chapter 13 l Fetal Orientation in Utero
Synclitism
The condition of parallelism between the plane of the pelvis and that of the fetal head.
Synclitism
| Posterior Asynclitism | Anterior Asynclitism |
Posterior parietal bone is over the inlet
Anterior parietal bone is over the inlet
Figure I-13-7. Synclitism
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Normal and Abnormal Labor 14
OVERVIEW OF LABOR
Labor is a process whereby over time regular uterine contractions bring about progressive effacement and dilation of the cervix, resulting in delivery of the fetus and expulsion of the placenta. Contractions will occur at least every 5 min lasting 30 s.
Physiology. Increasing frequency of contractions is associated with the formation of gap junc-tions between uterine myometrial cells. These events are correlated with increasing levels of oxytocin and prostaglandins along with multiplication of specific receptors.
Uterine Changes. The contractile upper uterine segment, containing mostly smooth musclefibers, becomes thicker as labor progresses, exerting forces that expel the fetus down the birth canal. The lower uterine segment, containing mostly collagen fibers, passively thins out with contractions of the upper segment.
Cervical Effacement. Cervical softening and thinning occur as increasing levels of oxytocin andprostaglandins lead to breakage of disulfide linkages of collagen fibers, resulting in increasing water content. Effacement is often expressed in percentages with the uneffaced (0%) cervix assumed to be 2 cm long and 2 cm wide. Progressive shortening and thinning lead to full efface-ment (100%) in which the cervix has no length and is paper-thin.
Cervical Dilation. This occurs as the passive lower uterine segment is thinned and pulled up bythe contractile upper segment. In early labor (latent phase), the rate of dilation is slow, but at 3–4 cm of dilation, the rate accelerates to a maximum rate in the active phase of labor. Complete dilation is expressed as 10 cm.
Cardinal Movements of Labor. The first 3 steps occur simultaneously.
• Engagement: movement of the presenting part below the plane of the pelvic inlet.
• Descent: movement of the presenting part down through the curve of the birth canal. simultaneous
• Flexion: placement of the fetal chin on the thorax.
The next 4 steps occur in order.
• Internal rotation: rotation of the position of the fetal head in the mid pelvis fromtransverse to anterior-posterior.
• Extension: movement of the fetal chin away from the thorax.
• External rotation: rotation of the fetal head outside the mother as the head passesthrough the pelvic outlet.
• Expulsion: delivery of the fetal shoulders and body.
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STAGES OF LABOR
Stage 1
This begins with onset of regular uterine contractions and ends with complete cervical dilation at 10 cm. It is divided into a latent and an active phase.
• Latent phase begins with onset of regular contractions and ends with the accel-eration of cervical dilation. Its purpose is to prepare the cervix for rapid dilation through effacement. Essentially no descent of the fetus occurs. Average duration of the latent phase is 6.4 hours in a primipara and 4.8 hours in a multipara. The upper limit of duration may be up to 20 h in a primipara and up to 14 h in a multipara. Abnormalities include prolonged latent phase.
• Active phase begins with cervical dilation acceleration, usually at 4–5 cm of dilation,ending with complete cervical dilation. Its purpose is rapid cervical dilation. The cardi-nal movements of labor occur, with beginning descent of the fetus in the latter part ofthe active phase. The rate of dilation is at least 1.2 cm/h in a primipara and 1.5 cm/h in a multipara. Abnormalities include prolonged active phase or arrest of active phase.
Stage 2
This begins with complete cervical dilation and ends with delivery of the fetus. Its purpose is descent of the fetus through the birth canal as maternal pushing efforts augment the uterine contractions. Duration may be up to 2 h in a primipara and 1 h in a multipara. Abnormalities include prolonged second stage or arrest of descent.
Stage 3
This begins with delivery of the fetus and ends with expulsion of the placenta. Placental sepa-ration from the uterine wall occurs as myometrial contractions shear off the anchoring villi. This is often augmented with IV oxytocin infusion. Signs of the third stage include gush of blood vaginally, change of the uterus from long to globular, “lengthening” of the umbilical cord. Duration may be up to 30 min in all women. Abnormalities include prolonged third stage.
Stage 4
This is a 2-h period of close observation of the parturient immediately after delivery. Vital signs and vaginal bleeding are monitored for onset of preeclampsia and postpartum hemorrhage.
Table 14-1. Stages of Labor
| Labor Stage | Definition | Function | Duration |
| Stage 1—Latent phase | Begins: onset of regular uterine contractions | Prepares cervix | <20 hours in primipara |
| Effacement | Ends: acceleration of cervical dilation | for dilation | <14 hours in multipara |
| Stage 1—Active phase | Begins: acceleration of cervical dilation | Rapid cervical | >1.2 cm/hours primipara |
| Dilation | Ends: 10 cm (complete) | dilation | >1.5 cm/hours multipara |
| Stage 2 | Begins: 10 cm (complete) | Descent of the | <2 hours in primipara |
| Descent | Ends: delivery of baby | fetus | <1 hour in multipara |
| Add 1 hour if epidural | |||
| Stage 3 | Begins: delivery of baby | Delivery of | <30 minutes |
| Expulsion | Ends: delivery of placenta | placenta | |
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Chapter 14 l Normal and Abnormal Labor
| 10 | |||||||||
| Dilation | 8 | ||||||||
| 6 | |||||||||
| Cervical | 4 | ||||||||
| 2 | Active | Second | |||||||
| Latent phase | phase | stage | |||||||
| 0 | |||||||||
| 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | ||
Time Hour
| Prolonged Latent Phase Labor | Prolonged Active Phase Labor | Protracted Arrest of Dilation | ||||||||||
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| Cervical dilation | Dilation |
| Dilation |
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| Time | Time | Time |
| Protracted Descent | Arrest of Descent | |
| Descent | Descent | |
| Time | Time | |
| Figure I-14-1. Stages of Labor | ||
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USMLE Step 2 l Obstetrics
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