Where is the flexion point of the fetal head located for vacuum cup placement during an assisted vaginal delivery?

Flexion Point Location for Vacuum Cup Placement

The flexion point is located 3 cm anterior to the posterior fontanelle (or equivalently, 6 cm posterior to the anterior fontanelle), making option "a" the correct answer. 1

Anatomical Definition

The flexion point represents the optimal location for vacuum cup placement to achieve proper flexion of the fetal head during assisted vaginal delivery. This specific landmark is critical for successful vacuum extraction:

• The flexion point is precisely defined as 3 cm forward (anterior) from the posterior fontanelle 1
• Alternatively described as 6 cm posterior to the anterior fontanelle 1
• This location corresponds to the pivot point around which the fetal head flexes during descent through the birth canal

Clinical Significance of Proper Cup Placement

Correct cup placement over the flexion point is essential for safe and effective vacuum-assisted delivery:

• Failures and complications in vacuum extraction are directly associated with not applying the vacuum cup to the flexion point on the fetal scalp 2
• Proper cup placement on the fetal head and knowing when to abandon the procedure are key components to conducting a safe and successful vacuum delivery 3
• High success rates (90% or greater) for vaginal delivery are achieved when flexing cup applications are correctly positioned 4

Techniques to Improve Accuracy

Research demonstrates that accuracy of cup placement can be enhanced through specific approaches:

• Intrapartum transabdominal ultrasound assessment significantly improves the accuracy of vacuum cup placement, reducing the mean distance from the flexion point from 2.8 cm (digital examination alone) to 2.1 cm (digital plus ultrasound) 1
• The Vacca 5-Steps technique achieves better cup placement than the Bird technique, particularly in transverse and posterior positions, with mean distances from the flexion point of 1.05 cm versus 1.78 cm respectively 5

Common Pitfalls to Avoid

Misplacement of the vacuum cup away from the flexion point leads to:

• Inadequate flexion or unintended hyperextension of the fetal head 6
• Increased risk of cup detachment and failed delivery 2
• Potential for fetal scalp trauma when the cup is incorrectly positioned 4
• The use of vacuum at cesarean delivery should be avoided entirely as it can cause significant fetal injury including intracranial and subgaleal hemorrhage 7