What is engagement of the fetal head?

Engagement of the Fetal Head

Engagement of the fetal head refers to when the widest diameter of the fetal head has passed through the pelvic inlet, with the leading part of the skull at or below the level of the ischial spines. 1, 2

Clinical Definition and Assessment

• Engagement is traditionally assessed by determining whether the widest part of the fetal skull has passed through the pelvic inlet 1
• Clinical diagnosis is typically made through both vaginal and abdominal examinations 2
• During vaginal examination, engagement is considered to have occurred when the presenting part has reached the level of the ischial spines 2
• Abdominal examination assesses the proximal part of the fetal head in relation to the pelvic brim 2

Importance in Labor Management

• Proper assessment of fetal head engagement is essential before attempting operative vaginal delivery 2, 3
• Engagement status helps predict the likelihood of successful vaginal delivery 2
• Incorrect assessment of engagement can lead to failed instrumental deliveries and increased maternal and neonatal complications 4

Challenges in Assessment

• Reliance solely on vaginal examination can be misleading, especially when molding of the fetal head is present 2
• In molded fetal heads, the leading part may be below the ischial spines while the widest diameter remains above the pelvic inlet 1
• Abdominal examination has been shown to be more accurate (94% vs 80%) than vaginal examination alone in predicting successful vaginal delivery 2

Modern Assessment Techniques

• Ultrasound provides objective measurements of fetal head descent and engagement 5
• Transperineal ultrasound can measure:
  • Angle of progression (cutoff of >120° suggests engagement) 4
  • Head-perineum distance (HPD) with 57mm as a cutoff distinguishing between engagement and non-engagement 3
• Transabdominal ultrasound can assess the suprapubic descent angle, which strongly correlates with transperineal measurements 1

Clinical Implications

• Uncertain fetal head engagement is associated with increased risk of postpartum hemorrhage and neonatal impairment 4
• When engagement status is uncertain, ultrasound measurement of the angle of progression may help reduce cesarean delivery rates 4
• Combining clinical examination with ultrasound assessment provides the most accurate determination of engagement status 3, 5

Warning Signs of Potential Complications

• Prolonged deceleration phase (from 8-10 cm dilation) may signal potential complications related to fetal descent 6
• When disorders of fetal descent are associated with prolonged deceleration phase, there is a higher risk of complications like shoulder dystocia 6
• In cases of impacted fetal head (where the head is wedged deep in the pelvis), special maneuvers may be required during cesarean delivery 7