Assessment of Ischial Spines During Pelvic Examination in Pregnancy
The ischial spines are palpated during digital vaginal examination by sweeping your examining fingers laterally along the pelvic sidewalls at approximately the mid-pelvis level, where you will feel bony prominences projecting into the pelvic cavity—these landmarks are critical for determining fetal head station.
Digital Pelvic Examination Technique
To locate the ischial spines during a vaginal examination, insert your examining fingers into the vagina and sweep laterally along both pelvic sidewalls at the level of the mid-pelvis. The ischial spines are felt as bony prominences that project medially into the pelvic cavity. 1
Key Anatomical Landmarks
The ischial spines represent station "0" in the traditional assessment of fetal head descent, serving as the reference point for measuring how far above (negative stations) or below (positive stations) the fetal presenting part has descended. 1
The interspinous diameter (distance between the two ischial spines) is the narrowest transverse diameter of the pelvis and represents the most common level where obstructed labor occurs. 2
Recent MRI-based research has established that the ischial spines are located at a specific angle from the pubic symphysis—the midline symphysis-ischial spine angle (mSIA) measures approximately 109.6° in pregnant women at term. 3
Clinical Significance During Labor Assessment
Fetal head station is assessed relative to the ischial spines, with the head at the level of the spines designated as station 0, above the spines as negative stations (-1, -2, -3), and below as positive stations (+1, +2, +3). 1
As the fetal head descends from station -1 to +2, there is a 25% increase in the anteroposterior diameter of the levator ani hiatus and a doubling of the hiatal area, demonstrating the progressive stretching of pelvic floor structures. 1
Common Pitfalls and Caveats
The ischial spines can be difficult to palpate, particularly in women with larger body habitus, during early labor when the presenting part is high, or when there is significant caput or molding of the fetal head.
Avoid confusing the ischial spines with the sacral promontory (felt posteriorly at the pelvic inlet) or the ischial tuberosities (felt more inferiorly and laterally at the pelvic outlet).
The fetal head position (occiput anterior, posterior, or transverse) does not significantly affect pelvic floor measurements at a given station, so focus on the relationship between the presenting part and the spines themselves. 1
Alternative Assessment Methods
Transperineal ultrasound can be used intrapartum to assess fetal head descent when digital examination is limited, though the ischial spines themselves cannot be directly visualized on ultrasound. 3, 1
The angle of progression measured on transperineal ultrasound at station 0 theoretically corresponds to approximately 109.6° based on MRI-derived anatomical data. 3
CT pelvimetry can measure the interspinous diameter accurately, though it requires radiation exposure and is rarely indicated in modern obstetrics; importantly, the fovea of the femoral head is an unreliable landmark for identifying the level of the ischial spines on imaging. 2