We used a simulation model to measure forces on the fetal neck during breech delivery. This is to the best of our knowledge the first analysis on the impact of birthing positions and maneuvers during breech delivery on the fetal neck. We could show that the applied force was lowest in uncomplicated deliveries in all-fours position. As soon as interventions were necessary, the force acting on the neck increased, as well as with the fetal weight and the number of maneuvers applied.
Several studies have indicated that vaginal breech delivery is one of the main risk factors for brachial plexus palsy due to complicated head delivery and traction on the fetal neck. The meta-analysis by Van der Looven et al. was performed to identify risk factors for permanent brachial plexus injury. Spontaneous breech delivery was a significant risk factor (OR 2.49; 95% CI 1.67–3.7; I2 = 70%). Lalka et al. described breech delivery as one of the strongest predictors for brachial plexus palsy (OR 15.38; 95% CI: 5.60, 42.25). These studies show that brachial plexus palsy is an issue in breech delivery and should become an integral part of future training for spontaneous breech delivery, potentially directly measuring forces applied in a simulation model.
The retrospective cohort study by Louwen et al.  investigated real-life deliveries with breech presentation in an upright versus lithotomy positions and compared them to elective cesarean section. They found fewer neonatal birth injuries and a decreased secondary cesarean section rate for delivering breech in an upright position versus lithotomy. The limitation of the study was the small sample size of the control group: 229 deliveries in the upright and 40 in the lithotomy presentation .
Most studies of breech deliveries are retrospective and based on registry data. This makes comparisons between the different delivery methods difficult.
We used for the first time a full body high fidelity obstetrical simulator (SimMom, Laerdal), to investigate the force applied on the fetal neck during delivery in fetal breech presentation. We did not find comparable studies evaluating the forces affecting the fetus during vaginal breech delivery. There are only studies analysing forces on the fetal head or the brachial plexus during cephalic presentation. Allen, Sorab and Gonik indirectly measured the forces, which were applied in real-life vaginal deliveries (cephalic presentation) by using force-sensing devices attached to the provider’s delivery hand. They found that forces were typically about 47 N for routine deliveries, 69 N for difficult deliveries and 100 N for shoulder dystocia. A single case was reported where a force of 99,89 N resulted in a transient brachial plexus injury . Another study was designed by Allen et al. . They developed a birthing model and a microcomputer data acquisition system. This was used to measure applied forces during vaginal cephalic delivery for routine, difficult and shoulder dystocia cases. The physicians obtained 84 N for routine deliveries, 122 N for difficult and 163 N for shoulder dystocia deliveries (p < 0.002). Force levels exceeding 100 N were reached for many physicians during emergency situations . The authors designed another trial and used a force-measuring birth simulator, which consists of a supported overhung beam and was equipped with sensors. Clinicians simulated again routine, difficult and shoulder dystocia deliveries . Average peak forces of 68 N for routine deliveries, 118 N for difficult and 172 N for shoulder dystocia were revealed .
The absolute numbers of the above mentioned three studies cannot be directly compared with ours, because they used indirect methods, whereas in our study the forces were directly measured within the fetal neck. Another difference of their study and ours is the fetal position (cephalic vs. breech presentation). However, measurements above 100 N should ideally be avoided as such high values were associated with emergency situations.
The lowest force in this simulation study could be recorded for the spontaneous delivery in all-fours position without the need of any further maneuver. We could demonstrate that by using any of the described maneuvers a higher impact of applied force was recorded. This can be explained by a higher tension on the fetal body during the maneuver, whereas in all-fours position within an uncomplicated delivery only gravity influences the applied force. As shown in Fig. 3B, the affecting force in all-fours position clearly depends on the fetal weight. If optimally performed, maneuvers in lithotomy position led to forces similar to those detected in uncomplicated all-fours position. However, as shown in Fig. 3B the performance of each maneuver depends on the conducting physician and shows a significantly higher variation regarding each individual performance compared to uncomplicated all-fours position. Therefore, maneuvers during breech delivery should only be applied if absolutely necessary.
It is interesting that, within the complicated deliveries, the Veit-Smellie-Mauriceau maneuver is associated with the lowest force on the fetal neck region. This can be explained by the fact that the physician directly grasps the fetal head and thereby prevents any traction on the fetal neck. On the other hand, potential soft tissue injuries of the oral region have to be considered, especially when an index finger is introduced into the mouth rather than two fingers placed on the cheek bones.
It was interesting to note that we detected the highest forces applied to the fetal neck region during cases of shoulder dystocia, for both all-fours and lithotomy position. However, the time point where the maximum force occurred was different for these two positions (Fig. 4C).
In lithotomy position we detected the highest forces while resolving the shoulder dystocia (mean value 127.98 N, SD 22.27 N). Interestingly, in all-fours position the maximum force was applied during the delivery of the head (mean value 116.78 N, SD 15.57 N). This is important information and should help us to specifically focus training on the most dangerous situations.
A strength of our study is the use of a high-fidelity obstetrical simulator and Birthing Baby including internal built-in sensors which made direct recordings possible. A further strength is the use of Automatic Delivery Module 2 capable of standardizing contraction forces. This simulation is capable to reflect almost any emergency situation. However, this model only provides the opportunity to measure forces applied to the fetal neck. We decided to use the SimMom simulator and Birthing Baby together with PROMPT Flex-Software, instead of the PROMPT hemi-pelvis. The advantage of the SimMom is the flexibility to simulate different birthing positions and to be able to put the simulator in all-fours. Additionally, this standardized model does not reflect individual patient characteristics such as anatomical differences, pelvic floor function and fetal size.
Our proposed simulation model and our systematic analyses can be used to better train and reproduce individual situations and systematically evaluate the impact of obstetrical maneuvers. Future research should concentrate on the demonstration of how the forces applied to the fetus change with training and how feedback can improve the individual performance, reduce damage and optimize conditions for safe vaginal delivery.