Abnormal labor, also referred to as dystocia, is influenced by factors such as the forces of labor, the birth canal, the fetus, and psychosocial factors. These factors interact with one another and can be both causes and consequences of each other. Abnormalities in any single factor or a failure of these factors to adapt to one another can obstruct the birthing process, which is then classified as abnormal labor.
During abnormal labor, early recognition is crucial, along with a comprehensive analysis of the forces of labor, the birth canal, the fetus, and psychosocial influences. For example, a narrow pelvis may lead to abnormal fetal positioning and uterine inertia, while uterine inertia can itself cause fetal malposition. Some conditions, such as uterine inertia and abnormal fetal positioning, may be correctable, potentially allowing progression to normal delivery. Identifying the underlying causes of abnormal labor, making accurate judgments, and providing appropriate management are essential for ensuring a smooth delivery and maternal-fetal safety.
Etiology
The most common causes involve abnormalities in the forces of labor, the birth canal, and the fetus.
Abnormalities in the Forces of Labor
These include various dysfunctions in uterine, abdominal, diaphragmatic, or levator ani muscle contractions, with the most common issue being uterine contraction abnormalities. Uterine contraction abnormalities are further categorized into hypocontractility (coordinated and uncoordinated) and hypercontractility (coordinated and uncoordinated).
- Hypocontractility may result in prolonged or arrested labor.
- Hypercontractility may lead to precipitous labor or serious complications.
Abnormalities of the Birth Canal
Abnormalities of the birth canal frequently lead to cephalopelvic disproportion and can involve issues with the bony birth canal or soft tissues. The most common issue is pelvic narrowing. Bony birth canal abnormalities involve abnormal pelvic size or shape, which results in shortened pelvic diameters and can be classified as narrow pelvis, pathological pelvis, or deformed pelvis.
Fetal Factors
Fetal factors include abnormal presentations (e.g., abnormal cephalic presentation, breech presentation, or shoulder presentation), relative fetal macrosomia, and fetal developmental abnormalities.
Clinical Manifestations
Abnormal fetal presentations, developmental abnormalities, severe narrowing of the bony birth canal, or abnormalities in the soft tissues are often identified during the prenatal period. However, most cases of abnormal labor manifest during the delivery process.
Maternal Manifestations
Systemic Signs of Maternal Exhaustion
Prolonged labor may cause maternal restlessness, physical exhaustion, and reduced oral intake. Severe cases may result in dehydration, metabolic acidosis, electrolyte imbalance, intestinal bloating, or urinary retention.
Obstetrical Signs
These may include uterine hypocontractility or hypercontractility and excessive frequency of contractions, cervical edema, or slow or arrested cervical dilation. Delayed or stagnant fetal descent may occur, and in severe cases, there may be extreme elongation of the lower uterine segment, pathological retraction rings, tenderness in the lower uterine segment, hematuria, uterine rupture warning signs, or even actual uterine rupture.
In cases of cephalopelvic disproportion or abnormal fetal positioning, a gap may be observed between the presenting part of the fetus and the pelvic bones. Amniotic fluid may flow between the anterior and posterior amniotic sacs, with unequal pressure on the anterior amniotic sac during uterine contractions. This can lead to premature rupture of membranes (PROM), which is often an indicator of abnormal labor and may warrant an evaluation for cephalopelvic disproportion or abnormal fetal presentation.
Fetal Manifestations
Failure of Fetal Head Engagement or Delayed Engagement
A persistently high and unengaged fetal head after the onset of labor, or failure of engagement when the cervix is dilated beyond 5 cm, suggests the possibility of significant cephalopelvic disproportion or fetal malposition.
Abnormal Fetal Positioning
Abnormal cephalic positions are a primary cause of cephalic dystocia. These may include abnormal fetal orientations at engagement (such as high straight position or asynclitism), failure of normal internal rotation (e.g., persistent occiput posterior or transverse positions), or fetal attitude abnormalities (e.g., deflexed head in sinciput, brow, or face presentations, or lateral head flexion in asynclitic presentations). These abnormalities hinder fetal descent, slow or arrest cervical dilation, and can lead to secondary uterine inertia.
Fetal Head Edema or Hematoma
Prolonged or arrested labor may cause soft tissue edema or hematoma at the presenting part of the fetal head due to prolonged compression or traction in the birth canal. Subperiosteal blood vessel rupture may occur, resulting in caput succedaneum or scalp hematoma.
Excessive Overlapping of Fetal Cranial Sutures
During labor, slight overlapping of the cranial sutures can reduce the size of the fetal head and facilitate delivery. However, in cases of prolonged labor due to a narrowed bony birth canal, excessive cranial suture overlapping can indicate significant cephalopelvic disproportion.
Fetal Distress
Prolonged labor, particularly during the second stage, can lead to fetal hypoxia. The fetus may experience reduced or lost compensatory capacity, resulting in signs of fetal distress.
Abnormal Progression of Labor
Abnormalities of the First Stage of Labor
These include a prolonged latent phase and abnormalities in the active phase.
Prolonged Latent Phase
The latent phase lasts from the onset of regular uterine contractions until the beginning of the active phase (cervical dilation at 5 cm). A latent phase lasting more than 20 hours in nulliparas or more than 14 hours in multiparas is defined as a prolonged latent phase.
Active Phase Abnormalities
These include a prolonged active phase and arrested active phase. The active phase is the period from cervical dilation of 5 cm to complete dilation. A rate of cervical dilation less than 0.5 cm/hour is termed a prolonged active phase. Arrested active phase occurs when cervical dilation stops for ≥4 hours despite adequate uterine contractions or for ≥6 hours if contractions are insufficient after the membranes have ruptured and cervical dilation is ≥5 cm.
Abnormalities of the Second Stage of Labor
These include protracted descent, arrested descent, and a prolonged second stage of labor.
Protracted Descent
Fetal descent during the second stage is considered protracted when the descent rate of the presenting part is <1 cm/hour in nulliparas or <2 cm/hour in multiparas.
Arrested Descent
Arrest of descent occurs when fetal descent ceases for more than 1 hour during the second stage of labor.
Prolonged Second Stage
The second stage is considered prolonged when it lasts more than 3 hours in nulliparas or more than 2 hours in multiparas. In cases of epidural analgesia, the duration thresholds increase to more than 4 hours for nulliparas and more than 3 hours for multiparas. Lack of progress (no descent or rotation of the fetal head) characterizes this condition.
Management
The primary principle is prevention. A comprehensive assessment should be conducted, including evaluations of uterine contraction strength, fetal size and presentation, pelvic dimensions, and the proportion of cephalopelvic relationships. Decisions regarding the mode of delivery should be based on these analyses. If available, intrapartum ultrasound may aid in assessing labor progress and the likelihood of delivery success.
Trial of Labor
If no evidence of significant cephalopelvic disproportion exists, a trial of labor is generally recommended. To avoid an arbitrary diagnosis of dystocia, the following considerations are important:
- Dystocia should not be diagnosed before cervical dilation has reached 4 cm during the first stage of labor.
- Dystocia should only be diagnosed after artificial rupture of membranes and oxytocin augmentation have failed to promote labor progress. During the trial of labor, if abnormalities occur, appropriate management depends on the specific situation.
Prolonged Latent Phase
Diagnosing prolonged latent phase is challenging due to difficulty determining the precise onset of labor. A prolonged latent phase is not an absolute indication for cesarean section. If the cervical dilation is 0–3 cm and the latent phase exceeds 8 hours, intramuscular pethidine (100 mg) may be administered to address uncoordinated uterine contractions, relieve the pain of contractions, and allow the patient to rest, potentially enabling transition to the active phase. If uterine contractions do not improve after sedative use, intravenous oxytocin infusion may be administered. For cervical dilation >3–5 cm with no progress in dilation over 2–4 hours, artificial rupture of membranes and oxytocin infusion may be used to strengthen uterine contractions and promote labor progression.
Active Phase Abnormalities
For prolonged active phase, thorough vaginal examination is needed to evaluate pelvic dimensions and fetal positioning. If there is no significant cephalopelvic disproportion or severe abnormality in fetal positioning, artificial rupture of membranes followed by intravenous oxytocin infusion may be performed to enhance uterine contractions and accelerate labor progress. For abnormal fetal positions, such as occiput transverse or occiput posterior positions, manual rotation may be attempted to correct the fetal position. Arrested active phase suggests cephalopelvic disproportion, warranting cesarean delivery.
Second Stage Abnormalities
Second stage abnormalities require careful consideration of the possibility of cephalopelvic disproportion. Assessment of maternal effort, fetal heart rate, fetal position, pelvic dimensions, fetal head station, fetal head edema, and cranial bone overlap is crucial. If cephalopelvic disproportion or severe fetal positioning abnormalities are absent, oxytocin may be administered to enhance uterine contractions, and maternal pushing efforts may be guided. For occiput transverse or occiput posterior positions, manual fetal head rotation to occiput anterior may be attempted. If the fetal head descends to a station of S≥+3, vacuum extraction or forceps-assisted delivery may be considered. If no progress occurs following these interventions and the fetal head remains at a station of S≤+2, cesarean delivery should be performed promptly.
Cesarean Delivery
Cesarean delivery should be performed immediately in cases of severe fetal malposition, such as persistent high straight fetal head position, asynclitism, brow presentation, or mentoposterior positioning, with cessation of trial of labor. Scheduled cesarean delivery is indicated for absolute pelvic narrowing, macrosomia, significant cephalopelvic disproportion, shoulder presentation, or breech presentation, particularly footling breech. For pathological retraction rings or signs of uterine rupture with abnormal uterine contractions, cesarean delivery should proceed regardless of fetal viability and efforts to suppress contractions. When fetal distress develops and the cervix has not fully dilated with the fetal head at a station above S+2, cesarean delivery should also be considered.