Significance of the Ischial Spine During Labour
The ischial spines serve as the critical anatomical landmark for determining fetal head station (zero station), which directly predicts labor progression, risk of cephalopelvic disproportion, and likelihood of successful vaginal delivery.
Anatomical Reference Point for Fetal Station
The ischial spines represent the zero station reference point in the traditional assessment of fetal descent during labor. 1, 2
Geometric relationship: The angle between the pubic symphysis and the midpoint of the ischial spines averages 99-110 degrees in the mid-sagittal plane, providing an objective reference for station assessment. 1, 3, 2
Distance measurement: The ischial spines are located approximately 26 mm posterior to the vertical plane passing through the lower edge of the pubic symphysis. 3, 2
Clinical stations: Each centimeter of fetal head position above or below the ischial spines (-5 to +5 station) corresponds to specific measurable angles that can be calculated using geometric models. 1
Clinical Significance for Labor Management
Prediction of Delivery Mode
Fetal head position relative to the ischial spines is the strongest predictor of whether vaginal delivery will succeed or cesarean section will be required. 4
When the fetal head is ≤40 mm from the perineum (corresponding to station at or below the ischial spines), 93% of women deliver vaginally. 4
When the fetal head remains >50 mm from the perineum (above the ischial spines), only 18% achieve vaginal delivery. 4
The predictive accuracy of ultrasound-measured fetal station relative to ischial spines (area under ROC curve 81%) significantly exceeds digital examination alone (66%). 4
Assessment of Cephalopelvic Disproportion
The relationship between the fetal head and ischial spines is essential for diagnosing CPD, which has critical implications for maternal and fetal morbidity. 5
Failure of descent past the ischial spines during active labor, particularly when combined with protracted active phase or prolonged deceleration phase, strongly suggests CPD. 5
Oxytocin augmentation should be avoided when the fetal head fails to descend past the ischial spines despite adequate contractions, as this indicates mechanical obstruction rather than inadequate contractility. 6
Attempting vaginal delivery when the head remains above the ischial spines with arrested descent risks uterine rupture, excessive fetal head molding, and intracranial injury. 5
Risk Stratification for Operative Delivery
Station relative to the ischial spines determines the safety and feasibility of operative interventions. 7
Deeply engaged fetal head (low station, well below ischial spines) creates risk of impacted fetal head during cesarean delivery, requiring special extraction maneuvers. 7
High station (above ischial spines) at full dilation increases risk of failed operative vaginal delivery and subsequent emergency cesarean with higher maternal morbidity. 5
Impact on Pelvic Floor Structures
The descent of the fetal head past the ischial spines causes progressive distension of maternal pelvic floor muscles with significant implications for long-term morbidity. 8
Levator ani muscle stretching: As the fetal head descends from station -1 to +2 (relative to ischial spines), the anteroposterior diameter of the levator hiatus increases by 25% (from 6.1 to 8.1 cm) and the hiatal area doubles (from 16.3 to 30.3 cm²). 8
Levator ani avulsions occur in 29% of women following vaginal delivery, but these injuries do not occur until the head passes below the ischial spines and delivers. 8
Cesarean section before descent past the ischial spines is not associated with levator avulsions, providing important counseling information for women considering mode of delivery. 8
Common Clinical Pitfalls
Inaccuracy of Digital Examination
Digital assessment of fetal station relative to the ischial spines correlates poorly with actual anatomical position, especially at stations below zero, where accurate assessment has the greatest clinical impact on decision-making. 1
The ischial spines are difficult to palpate reliably during digital examination, particularly in women with small pelvis, obesity, or when the fetal head is deeply engaged. 5
Cervical examination may be impossible in women with skeletal dysplasia due to pelvic anatomy, making assessment of station relative to ischial spines unreliable and potentially dangerous if labor management decisions are based on incomplete information. 5
Misinterpretation of Labor Progress
Failure to accurately identify when the fetal head reaches the level of the ischial spines (zero station) leads to misdiagnosis of labor abnormalities. 5
Prolonged deceleration phase (from 8-9 cm to complete dilation) is strongly associated with failure of the head to descend past the ischial spines and predicts second stage abnormalities and shoulder dystocia. 5
The combination of prolonged deceleration phase with failure of descent past the ischial spines makes safe vaginal delivery very unlikely and should prompt consideration of cesarean delivery before entering the second stage. 5
Practical Assessment Strategy
Use transperineal ultrasound to objectively measure fetal head position relative to the ischial spine level rather than relying solely on digital examination. 4, 1, 3, 2
Measure the angle of progression: An angle ≥110° indicates the head is at or below the ischial spines with 87% vaginal delivery rate. 4
Measure head-perineum distance: Distance ≤40 mm correlates with station at or below the ischial spines. 4
The symphysis-ischial spine angle of 99-110° on ultrasound corresponds to zero station and can be used to calculate other stations. 1, 3, 2