How is the pelvic inlet checked for delivery?

Assessment of the Pelvic Inlet for Delivery

The pelvic inlet is primarily assessed through clinical pelvic examination during labor, with imaging reserved for specific high-risk situations where cephalopelvic disproportion is suspected or when maternal skeletal dysplasia is present.

Clinical Assessment Methods

Physical Examination During Labor

• Digital vaginal examination remains the primary method for assessing pelvic adequacy, performed during active labor to evaluate the relationship between the fetal head and maternal pelvis 1
• Serial suprapubic palpation of the fetal skull base should be combined with digital examination to differentiate true fetal descent from molding, providing more accurate assessment of cephalopelvic relationship 1
• Clinical assessment focuses on identifying signs of cephalopelvic disproportion, including failure of descent despite adequate contractions and prolonged deceleration phase 1

Key Clinical Parameters to Evaluate

• Pelvic outlet dimensions are critical, as a narrow pelvic outlet (mean 328 mm versus 346 mm in successful vaginal deliveries) significantly increases emergency cesarean section risk 2
• Pelvic inlet measurements also matter, with mean inlet of 245 mm in cesarean cases versus 255 mm in vaginal deliveries 2
• The pelvic inlet area shows significant differences between delivery modes, with cesarean section group averaging 126.3 cm² compared to 134.9 cm² in vaginal delivery group 3

Imaging Modalities When Indicated

Ultrasound Assessment

• Vaginosonography can measure the true conjugate and transverse diameter of the pelvic inlet in suspected cephalopelvic disproportion cases, offering a quick, painless alternative to radiological methods 4
• This technique allows measurement of both anteroposterior and transverse pelvic dimensions without radiation exposure 4

MRI Pelvimetry

• Magnetic resonance pelvimetry provides detailed assessment of pelvic capacity across pregnancy, showing that pelvic dimensions increase from gestational week 20 to 32 in all three planes 5
• MRI demonstrates that maternal positioning affects pelvic capacity: supine position optimizes pelvic inlet size, while semi-lithotomy and kneeling squat positions increase mid- and outlet-pelvic capacities by up to 1 cm 5

X-ray Pelvimetry

• Postpartum X-ray pelvimetry is recommended after emergency cesarean section for protracted labor to guide route of delivery in future pregnancies 2
• This modality provides precise measurements of pelvic inlet and outlet dimensions for future pregnancy planning 2

Special Populations Requiring Different Approach

Maternal Skeletal Dysplasia

• Women with skeletal dysplasia have altered pelvic anatomy that precludes vaginal delivery, and cesarean delivery is recommended regardless of pelvic measurements 6, 1
• The infant's cranium will be too large to pass through the birth canal whether the infant has normal stature with normocephaly or short stature with macrocephaly 6, 1
• Cesarean delivery can be performed with Pfannenstiel skin incision and low transverse uterine incision in these patients 6

Critical Decision Points

When to Proceed with Cesarean Delivery

• If cephalopelvic disproportion is found or cannot be ruled out with reasonable certainty, cesarean delivery is the safer choice 1
• Never attempt oxytocin augmentation when cephalopelvic disproportion cannot be excluded, as this risks uterine rupture and maternal-fetal injury 1
• Prolonged deceleration phase combined with failure of descent makes safe vaginal delivery very unlikely 1

Fetal Factors Affecting Assessment

• Macrosomia is a specific risk factor for cephalopelvic disproportion requiring careful pelvic assessment 1
• Occiput posterior position increases operative delivery complications and signals underlying cephalopelvic disproportion 1
• Brow presentation is an absolute contraindication to vaginal delivery 1

Common Pitfalls to Avoid

• Do not rely solely on digital examination—always combine with suprapubic palpation to assess true descent versus molding 1
• Avoid prolonged labor trials when anatomical factors suggest disproportion, as risks of maternal and fetal damage are substantial 1
• Do not perform routine imaging pelvimetry in low-risk pregnancies, as clinical assessment during labor is sufficient for most cases 2
• Never ignore maternal skeletal dysplasia as a contraindication to vaginal delivery, regardless of other favorable factors 6, 1