A primigravida at term is in active labor with a fully dilated cervix and the fetal head at station +2. After 2 hours of pushing with no descent, the obstetrician decides on operative vaginal delivery. Which single feature best distinguishes vacuum extraction from forceps delivery in terms of mechanism of traction?

Explanation

## Mechanism of Traction: Vacuum vs. Forceps **Key Point:** The fundamental difference between vacuum and forceps lies in HOW traction is applied to the fetal head. ### Vacuum Extraction - Applies **negative pressure** (0.2–0.8 kg/cm²) to create a chignon (caput succedaneum) - Traction is indirect — force is transmitted through the scalp to the underlying skull - Allows **progressive descent** with each contraction and maternal push - Safer for the fetal head in terms of direct compression injury ### Forceps Delivery - Uses **direct mechanical grip** via two curved blades that articulate around the fetal head - Traction is direct — force applied to the biparietal diameter and occiput - Allows immediate **rotational control** (up to 180° for rotational forceps) - Greater risk of facial trauma, cephalohematoma, and intracranial injury if applied incorrectly **Clinical Pearl:** Vacuum is gentler on the fetal scalp but carries a higher risk of scalp lacerations and cephalohematomas. Forceps offer better control but risk facial bruising and skull fractures. **High-Yield:** The mechanism difference explains why vacuum is preferred for descent in low/mid-cavity positions, while forceps are preferred for rotation and when precise control is needed. | Feature | Vacuum | Forceps | |---------|--------|----------| | **Mechanism** | Negative pressure (indirect) | Direct mechanical grip | | **Scalp injury risk** | Chignon, cephalohematoma, lacerations | Facial bruising, skull fractures | | **Rotational capability** | Limited (45° safe) | Extensive (180° rotational) | | **Descent mechanism** | Progressive with contractions | Immediate with traction |