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

Optimal Vacuum Cup Placement: Location of the Flexion Point

The flexion point is located 3 cm anterior (forward) to the posterior fontanelle, making option (a) the correct answer.

Anatomical Definition of the Flexion Point

The flexion point represents the pivot point on the fetal skull where optimal flexion occurs during descent through the birth canal. 1

• The flexion point is precisely defined as 6 cm posterior to the anterior fontanelle OR 3 cm anterior to the posterior fontanelle 1
• These measurements are equivalent anatomical landmarks describing the same location on the fetal vertex 1

Clinical Significance of Proper Cup Placement

Accurate placement of the vacuum cup over the flexion point is critical for successful assisted vaginal delivery and prevention of complications. 2

• Misplacement away from the flexion point leads to inadequate flexion or unintended hyperextension of the fetal head 2
• Failures and complications in vacuum-assisted delivery are directly associated with not applying the vacuum cup to the flexion point on the fetal scalp 3
• When flexing cup applications were achieved in 90% of vacuum attempts, success rates for vaginal delivery reached 98% 4

Techniques to Improve Accuracy

The Vacca 5-Steps technique demonstrates superior accuracy compared to other placement methods, particularly in transverse and posterior positions. 5

• The Vacca technique achieved a mean distance of only 1.05±0.60 cm from the flexion point, compared to 1.78±0.96 cm with the Bird technique 5
• Intrapartum transabdominal ultrasound assessment of fetal head position significantly improves placement accuracy, reducing the mean distance from 2.8±1.0 cm (digital examination alone) to 2.1±1.3 cm (digital examination plus ultrasound) 1

Common Pitfall to Avoid

The most critical error is placing the cup too far anteriorly (toward the anterior fontanelle) or too far posteriorly (beyond 3 cm from the posterior fontanelle), which prevents proper flexion and increases failure rates. 3, 4

• Correct cup placement and knowing when to abandon the procedure are the key components to conducting a safe and successful vacuum delivery 6