Pregnancy reshapes the biomechanics of everyday safety systems, and a new observational study from the University of British Columbia (UBC) suggests that conventional seatbelt geometry often fails to land where it should. Researchers found that nearly 9 in 10 pregnant participants could not achieve recommended belt placement, even after they received instruction and hands-on guidance. Out of 333 participants, only 11.4% placed the belt according to guidelines.
Seatbelts are engineered to route crash forces through the body’s stronger skeletal framework. When the belt rides too high or shifts laterally, more fragile soft tissues and anatomy-altering regions may absorb greater loads, potentially raising injury risk during collisions. In the study, the most consistent challenge was the shoulder belt position rather than the lap belt.
Using a performance metric defined by national recommendations, the researchers evaluated whether the shoulder belt sat between the breasts and centered on the shoulder, while the lap belt remained below the abdomen and snug across the hips and pelvis. Most participants succeeded with lap-belt placement, but few managed to keep the shoulder belt between the breasts as pregnancy advanced.
As abdominal volume increases, the belt’s contact points can migrate. The study observed that the shoulder belt frequently shifts off the chest center and can ride across the breast region instead of maintaining the intended path. This matters because restraint effectiveness depends on the belt’s trajectory relative to the occupant’s changing anatomy.
The research also identified “nesting,” a failure mode in which the lap belt folds into softer tissue beneath the abdomen rather than lying flat. Nearly one-third of participants experienced nesting, and its frequency rose with gestational stage, indicating that belt fit is not static throughout pregnancy.
To capture these dynamic fit problems, the team scanned participants using handheld 3D imaging tools while they sat in a production vehicle seat equipped with a standard belt configuration. Participants ranged from 6 to 38 weeks pregnant and represented a variety of body types, improving the study’s relevance to real-world diversity.
UBC researchers argue that these findings highlight a broader gap in vehicle safety research: human body models used for restraint design have historically underrepresented female and especially pregnant anatomy. The study therefore points toward the need for restraint systems that can accommodate anatomy changes rather than assuming a single static body shape.
Rather than questioning the importance of seatbelt use, the authors emphasize that correct use remains life-saving. Seatbelts reduce the risk of death and serious injury in crashes, and guidance should be strengthened by better engineering and usability evidence.
The work is a collaboration between UBC and the Toyota Collaborative Safety Research Center, and the team is now partnering with Autoliv to build computational models of pregnant anatomy and test future restraint concepts virtually. The findings were presented at the 10th World Congress of Biomechanics in Vancouver.
Subject of Research: People
Article Title: Pregnancy alters seatbelt fit; most participants fail recommended placement in study
Web References: https://www.nhtsa.gov/vehicle-safety/seat-belts
References: https://tc.canada.ca/en/road-transportation/publications/archived/road-safety-canada#s31
Image Credits: Emma Fennell/UBC Applied Science
Keywords
Biomechanics; Automobile design; Automotive engineering; Seatbelt fit; Pregnancy safety; 3D body scanning; Restraint systems; Crash safety; Human factors
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