Documentation:FIB book/Seatbelt

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Overview

Summary

Pregnant People

Introduction

Various studies agree that pregnant people in motor vehicle accidents (MVAs) are at significantly higher risk of adverse effects but show conflicting results on how well seatbelts help protect the person and their fetus.

Generally, MVAs are a leading cause of traumatic fetal death and most agree that seatbelts should be properly worn by pregnant people to reduce chances of injury [1][2][3][4]. Articles also agree that the proper way of wearing a seatbelt while pregnant should be taught in prenatal counseling: “the correct way to wear a 3-point restraint during pregnancy is with the lap belt as low as possible below the bulge of the abdomen and the shoulder strap crossing diagonally above the bulge, ideally lying comfortably between the breasts” [4]. However, these studies also show that pregnant people often avoid wearing seatbelts due to various reasons regarding comfortability, especially in the third trimester, as well as fear that the belt would harm the fetus, which has been confirmed to be true if worn improperly [2][4].

Various ways of wearing a seatbelt while pregnant. Figure A depicts the correct way.

The CDC reports that 8% of people who were involved in a car crash and arrived at the emergency room have had to have been hospitalized in the general population of America [5]. To compare, a study by Whitehead et al. [1] shows that over 16% of pregnant people involved in an MVA had to be hospitalized. However, this study found no association of prenatal seatbelt counseling with hospitalization after a crash but did not look at how many of those who were being counselled on the correct way to wear a seatbelt did indeed wear one. A similar study done in Kuwait by Chibber et al. [2] agreed with the lack of association between counselling and hospitalization but concluded that most injuries severely affecting the fetus and mother occurred in those in their third trimester who were not wearing seatbelts. The study still suggested that all pregnant people should receive proper seatbelt counseling from prenatal care providers and that the uncertainties around the safety and use of seatbelts during pregnancy should be “dispelled”, recognizing that “it is important that pregnant [people] wear their seatbelt to prevent secondary collision with interior structures of the vehicle or sudden ejection to dissipate the force of an impact”. However, the study also raised the possibility that “perhaps not wearing [a] seatbelt rather than wrongly wearing the belt lowered the risk of uterine rupture in this study” [2].

An older study by Hyde et al. [3] conducted in the US also supports seatbelt use while pregnant, stating that “belt use has been shown to be effective in reducing morbidity and mortality for the mother and the fetus when comparing belted and unbelted pregnant women”. In addition, this study also found that “belted pregnant women in crashes were not significantly more at risk for adverse fetal outcomes than pregnant women not in crashes”. However, their data also shows that the risk of certain complications, including placental abruption and early delivery, are not reduced, and may even be increased by seatbelt usage. This was similar to Chibber et al.’s statement about the increased risk of uterine rupture with incorrect seatbelt usage [3]. No further studies were found comparing the outcome of pregnancies of those in crashes versus those not in crashes since this study from 2002. The lack of comparable and recent studies, as well as these conflicting results regarding this major issue, suggest that further studies must be done to not only find the effectiveness and of seatbelts for pregnant people, but also to explore alternative safety measures that are better designed for the very different anatomy of a pregnant person.

Current Studies and Common Injuries

Pregnant people can encounter several injuries due to current seatbelt designs in a possible accident. The following three subsections reflect experiments and studies around abdominal, chest, and neck injury areas as a result of a vehicle accident when the pregnant person was wearing a restraint, specifically a seatbelt. Each subsection includes information from the various paper’s injury loadings, methods, and results.

Abdominal Injuries

Studies show the leading cause of trauma in pregnant people of industrialized countries, experienced by 3 to 7% of that population, are car crashes [6]. Accidents on the road involve both the pregnant person and the fetus and cause adverse physiological and psychological outcomes. Abdominal injuries from car crashes have a great public health concern due to their socio-economic implications, and these injury risks must therefore be better understood [6].

Comparison of the load of the lap belt over time for the MAMA IIB (pregnant ATD) and the Hybrid III (non-pregnant ATD) during a 29km/hr frontal collision crash.

In a comparison study between pregnant and non-pregnant rear-seat passengers using the Maternal Anthropometric Measurement Apparatus dummy, version 2B (MAMA-2B) and the Hybrid III as back seat passengers, Kuwahara et al. analyzed the kinematics and abdominal forces during frontal collision sled tests at 29 and 48 km/hr with a three-point seatbelt to contrast injury features [7]. They determined that the belt loads were higher for the pregnant dummy compared to the Hybrid III due to the former’s higher body mass index. The difference in lap belt loading was nearly twice as high for the 48 km/hr test compared to the 29 km/hr one, jumping from around 1 kN to 2 kN. Therefore, this study suggests that the abdominal forces experienced by pregnant people for a given velocity are higher than for non-pregnant people. Moreover, the risk of negative fetal outcomes increases for restrained pregnant people in the back seat of a car with an increase in collision velocity [7]. For convenience and simplicity, this study only used a pregnant dummy of 30 weeks, meaning that the ATD’s biofidelity and the estimated forces may not be representative of all pregnant people, whose abdominal sizes differ greatly depending on the stage of pregnancy. Although this study illustrates the higher injury risk for pregnant people of 30 weeks gestation and poses caution to driving at high velocities, future research could be conducted to address the limitation of this study by determining how the stage of pregnancy can affect abdominal injury[7].

The study above outlines that abdominal injury risks caused by belt loads in car crashes are greater for pregnant passengers, but the effects of seatbelt use on pregnant drivers must also be understood. Motozawa et al. studied injury mechanisms of pregnant drivers using the MAMA-2B by conducting and analyzing both frontal and rear impact tests, with and without seatbelt usage[8]. To analyze the relationship between abdominal pressure and the change in velocity during vehicle collision, they proposed a risk curve to predict the adverse fetal outcome based on findings from previous research. Applying the risk curve to their sled test data, the probability of adverse fetal outcome was determined to be 20% in the belted tests for both front and rear impact, and 60% in the unbelted tests for both front and rear impact. During the frontal impact without a seatbelt, contact with the steering wheel caused peak abdominal pressure, which was avoided when the seatbelt was worn. Similarly, during the rear impact tests without a seatbelt, secondary contact with the steering wheel after a forward rebound movement caused an increase in abdominal pressure. Therefore, this study concluded that the use of a 3-point seatbelt reduces the risk of adverse fetal outcomes for pregnant drivers by preventing contact with the steering wheel during the crash[8]. Although Kuwahara et al. found that seatbelts may be less effective on pregnant people, this study’s findings confirm that using a seatbelt reduces the injury risk during collision, and that pregnant people should therefore be encouraged to use the 3-point seatbelt[7][8]. Similarly to Kuwahara al.’s study, this paper is limited by the use of a 30-week pregnant dummy, which may not be representative of the wide range of pregnant people. However, this study’s conclusions can be reasonably validated and correlated with other vehicle accident studies, where it is concluded that wearing a seatbelt increases safety[8].

ATD testing is advantageous when repeating tests multiple times, as done in the two studies described above, because of its convenience and relative ease of acquiring measurements[7][8]. However, dummies are only estimations of the human’s musculoskeletal response. A study by J. Delotte et al. acknowledged the lack of biofidelic pregnant models and aimed to increase analysis accuracy by examining the biomechanics of pregnant women in frontal impacts using a “physical model of the gravid uterus” inserted into the pelvis of a PMHS, which was fitted with 3D accelerometers[6]. MRI scans of a full-term pregnant person were used to determine the 3D geometry of the uterus and the model was constructed using silicon in a mold. A baby doll was inserted into the physical model and the remaining space was filled with water to resemble the amniotic fluid[6]. Regarding abdominal pressure from lap belt loading, the relative displacement of the fetal head depicted in the tests could give reason for vascular pelvic pedicle injuries. However, no intra-abdominal injury was seen during the post-experiment autopsy[6]. A limitation of the study is the lack of internal body pressure consideration which could relate to the pre-tensed state of the abdominal wall[6]. The authors seek to include “pressure transducers and diaphragm injury analysis” in further experiments to address this limitation. Overall, this paper recognizes the need for developing more comprehensive physical pregnant models, and the development of safety systems in vehicles for pregnant people, which aligns with the findings from Kuwahara et al.[6][7].

To address some of the limitations of using a simplified physical uterus as mentioned by J. Delotte et al., computational models can be used to represent a pregnant person’s biofidelity more accurately[6]. Serpil Acar et al. simulated dynamic responses due to acceleration impacts of a 5th percentile female occupant that’s 38 weeks pregnant, using the Mathematical Dynamic Models software for computational analysis [9]. The goal was to determine the effectiveness of seatbelts and airbags. Model validation was conducted to ensure an “accurate response of the pregnant abdomen, ” which helped ensure the biofidelity of the crash models. 21 sets of crash tests were conducted for frontal impact collisions ranging from 15 to 45 km/hr, at 5 km/hr intervals. All tests were conducted under the three following seatbelt conditions: completely unrestrained, using a three-point seatbelt, and using a three-point seatbelt with an airbag. When completely unrestrained, the abdominal impact with the steering wheel caused high strains in the placenta. When restrained with a three-point seatbelt, the fetus moved forward relative to the uterus due to the moment of inertia during impact, which created stress concentrations in the uterine wall where the fetus hits upon impact [9]. The results of this computational model align with Motozawa et al. who studied seatbelts with ATDs: a three-point seatbelt is effective at reducing the risk of abdominal injury[8][9]. However, without the use of an airbag, contact with the steering wheel still occurred at higher speeds which poses a greater risk of adverse fetal outcomes [8][9]. A three-point seatbelt with an airbag had the lowest risk of abdominal injury since the reduction in forward motion significantly reduced contact with the steering wheel [9]. Computational models can limited in their biofidelic responses because their inputs rely on mechanical properties that must be accurately determined through physical testing. However, this study appropriately aligns with previous research and raises awareness of the importance of using seatbelts and airbags for pregnant people[9].

Contrast-enhanced magnetic resonance imaging showing the placental region compressed by the seatbelt as a low-intensity band without enhancement. Retrieved from Yamada et al. case study.

Lastly, although using a seatbelt and an airbag is recommended to reduce abdominal injuries, seatbelt positioning on the body is also an important consideration to prevent adverse fetal outcomes[7][9]. Yamada et al. studied a case of a 26-year-old pregnant person involved in a motor vehicle accident that lead to fetal death due to “relatively mild seatbelt injuries” [10]. This crash system involved a rollover caused by an incoming vehicle at 50 km/hr, where both a three-point belt and an airbag were used. MRI findings concluded that the intrauterine fetal death occurred due to placental ischemia and placenta abruption because of seatbelt compression. Overall, this study blames improper seatbelt positioning as the prime reason for this injury and states that the lap belt should be placed “as low as possible under the pregnant abdomen” [10]. This study suggests that educating pregnant people on seatbelt placement, no matter the stage of pregnancy, should be required in prenatal care programs[10].

In summation, ATDs, PMHS crash tests, computational models, and real-life injury post-mortem imaging have all been used to determine the safety of pregnant people in vehicle crashes[6][7][8][9][10]. Unfortunately, there is a higher abdominal injury risk for pregnant people than for non-pregnant people[7]. However, some methods to reduce this risk are to: always wear a seatbelt[7][8]; use a 3-point seat belt used in conjunction with an airbag[8][9]; and place the seatbelt under the abdomen to ensure the safety of both the pregnant person and the fetus[10]. In general, more research is needed to fully understand the biofidelic response of a pregnant person’s abdomen at various different stages of pregnancy to help ensure the safety of all pregnant people during a car crash[6][7][8][9].

Neck Injuries

Another critical region of injury commonly caused by seatbelts in pregnant people is the neck. There are several possible neck injuries due to tension and compression as defined by the injury criteria. Hitosugi et al. conducted a study on shorter pregnant people restrained in the rear seat of a car through the use of pregnant crash test dummies and concluded that they are at higher risk of serious injury [11]. Pressure on the neck from the seatbelt, as well as the kinematics of the neck, head, and chest, was measured using the MAMA-2B. This took place during a full frontal impact sled test that was performed at a low speed of 29 km/hr and a medium speed of 48 km/hr with the dummy positioned in the rear seat. The primary risk of injury was found to be due to neck tension and compression while the secondary injury was assessed to be head injury [11].

After impact representation of the MAMA-2B dummy

According to Hitosugi et al.’s [11] test results, “the shoulder strap compressed the neck [with] a pressure >12.8 MPa even at low-velocity impact”. They also found that during the head’s lateral movement, “the shoulder strap path across the crash test dummy was in a high position”. Combining the head movement with the restricted movement of the right side of the chest (due to the seatbelt), both body parts moved in different directions on the coronal plane resulting in significant differences between the chest and head positions. From the article, the measured value differences are “a maximum of 42.4 mm at high velocity and 33.7 mm at low velocity”. The shoulder strap’s compression of the neck also caused a head moment, causing right-side bending as well as neck flexion. Specifically for people in the late term of their pregnancy, the belt deviates to the right or left to avoid crossing the enlarged abdomen likely causing the shoulder strap to make contact with the neck area [11].

Hitosugi et al.'s [11] study also examined the kinematic sequence of the pregnant dummy showing various time steps after impact initiation. It was demonstrated that the shoulder strap's path across the crash test dummy had an impact on the ATD's neck. This neck impact can be seen around 80 ms after crash initiation. It was also recorded that during a high-velocity impact, the dummy moved the furthest forward (164.1mm) which is a large amount likely to cause a neck injury as illustrated in the provided figure on the right. These results confirm that the neck is another critical and common region of injury in car accidents for pregnant people wearing seatbelts. As a solution for preventing these types of injuries, the study suggested that short pregnant people be raised using a booster seat to achieve the correct seatbelt path [11].

Chest Injuries

There have not been many experiments surrounding the effects of chest injuries on fetal outcomes due to seatbelt usage; nonetheless, a paper from Japanese experts A. Ishiko et al. examined chest compression of pregnant women due to seatbelts during vehicle frontal collisions using the MAMA-2B [12]. The size of this anthropometric device is based on the 5% percentile American female dummy and aimed to represent people in the later stages of pregnancy - about 30 weeks of gestation. During that time, the uterus enlarges and the uterine fundus begins to interface with the lower part of the chest, which gives rise to the importance of analyzing the biomechanics involved with chest impacts, and their relation to fetal outcome. To conduct experiments, the seating position of the dummy was determined, and the team used the Instron Servo Sled Apparatus to recreate a frontal crash, and the Infrared Telescoping Rob for Assessment of Chest Compression to measure chest compression [12]. Results showed that the dummy moved forward during the initial impact, causing chest compression to reach values up to 5.0 kN at a collision velocity of 26 km/hr from the three-point seatbelt. Moreover, chest deflection reached a maximum of 35.4mm on the left side, also at a collision velocity of 26 km/hr, but the side of deflection depended on the seatbelt positioning [12]. The paper concluded that negative fetal outcomes may be a result of “minor to moderate frontal collisions” due to “chest compression and subsequent applied forces to the uterus”, and suggests that further development of the seatbelt system may aid in reducing the applied forces on the chest and increase fetal safety [12]. This suggestion is similar to other studies in the literature review that also urge for more research and development on seatbelt systems for pregnant people.

Conclusion

Overall, most studies show the importance of further research and development in seatbelt use for pregnant people by stating that MVAs are a significant cause of morbidity and mortality to pregnant people and their infants. Although the belt does reduce the chances of severe injuries associated with high-energy MVAs, the effectiveness of the three-point seatbelt when worn by pregnant people is still not ideal for preventing injury to the fetus and mother [2][4]. Studies also recommend further research on the impact of prenatal seatbelt counselling and the proper use of seatbelts during pregnancy. They present inconsistent results regarding the effectiveness of seatbelts for pregnant people and there is still a shortage in recent studies [1][2][3]. Lastly, as indicated by the research papers depicted above, the main regions of potentially life-threatening injury tend to be the abdomen, chest, and neck, especially in shorter people, which pose different risks to the fetus and carrier through different mechanisms of injury. Studies exploring these injuries and risks are often limited by the lack of biofidelity and diversity in pregnant ATDs and comprehensive pregnant models, and there is also a shortage of studies, especially on the effects of chest injuries on fetal outcomes due to seatbelt usage.

References

  1. ↑ 1.0 1.1 1.2 N. S. Whitehead, “Prenatal Counseling on seatbelt Use and Crash-Related Medical Care,” Matern Child Health J, vol. 17, no. 9, pp. 1527–1532, Nov. 2013, doi: 10.1007/s10995-011-0861-2.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 R. Chibber, J. Al-Harmi, M. Fouda, and E. El-Saleh, “Motor–vehicle injury in pregnancy and subsequent feto-maternal outcomes: of grave concern,” The Journal of Maternal-Fetal & Neonatal Medicine, vol. 28, no. 4, pp. 399–402, Mar. 2015, doi: 10.3109/14767058.2014.918094.
  3. ↑ 3.0 3.1 3.2 3.3 L. Hyde, L. J. Cook, L. M. Olson, H. B. Weiss, and J. M. Dean, “Effect of motor vehicle crashes on adverse fetal outcomes,” Obstetrics & Gynecology, vol. 102, no. 2, pp. 279–286, Aug. 2003, doi: 10.1016/S0029-7844(03)00518-0.
  4. ↑ 4.0 4.1 4.2 4.3 S. K. Cesario, “seatbelt Use in Pregnancy: History, Misconceptions and the Need for Education,” Nursing for Women’s Health, vol. 11, no. 5, pp. 474–481, Nov. 2007, doi: 10.1111/j.1751-486X.2007.00215.x.
  5. ↑ “CDC report shows motor vehicle crash injuries are frequent and costly,” Centers for Disease Control and Prevention, 07-Oct-2014. [Online]. Available: https://www.cdc.gov/media/releases/2014/p1007-crash-injuries.html. [Accessed: 10-Nov-2022].
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 J. Delotte, M. Behr, L. Thollon, P.-J. Arnoux, P. Baque, A. Bongain, and C. Brunet, “Pregnant woman and road safety: Experimental crash test with post mortem human subject,” Surgical and Radiologic Anatomy, vol. 30, no. 3, pp. 185–189, 2008.
  7. ↑ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 7.10 A. Kuwahara, M. Hitosugi, A. Takeda, S. Tsujimura, and Y. Miyata, “Comparison of the injury mechanism between pregnant and non-pregnant women vehicle passengers using car crash test dummies,” 2022, vol. 10, no. 5, p. 884.
  8. ↑ 8.00 8.01 8.02 8.03 8.04 8.05 8.06 8.07 8.08 8.09 8.10 Y. Motozawa, M. Hitosugi, T. Abe, and S. Tokudome, “Effects of seatbelts worn by pregnant drivers during low-impact collisions,” American journal of obstetrics and gynecology, vol. 203, no. 1, pp. 62-e1, 2010.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 B. Serpil Acar and D. Van Lopik, “Computational pregnant occupant model,‘Expecting’, for crash simulations,” Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, vol. 223, no. 7, pp. 891–902, 2009.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 S. Yamada, K. Nishijima, J. Takahashi, N. Takahashi, C. Tamamura, and Y. Yoshida, “Intrauterine fetal death caused by seatbelt injury,” Taiwanese Journal of Obstetrics and Gynecology, vol. 56, no. 4, pp. 558–560, 2017.
  11. ↑ 11.0 11.1 11.2 11.3 11.4 11.5 Hitosugi, Masahito, et al. "Shorter pregnant women restrained in the rear seat of a car are at risk for serious neck injuries: Biomechanical analysis using a pregnant crash test dummy." Forensic science international 291 (2018): 133-137.
  12. ↑ 12.0 12.1 12.2 12.3 A. Ishiko, M. Hitosugi, M. Takaso, K. Mizuno, S. Tsuji, T. Ono, F. Kimura, and T. Murakami, “Chest compression of a pregnant woman by a seatbelt might affect fetal outcome, even in minor to moderate frontal vehicle collisions,” Forensic Science International, vol. 302, p. 109888, 2019.

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