Objective: A program was developed to study the mechanisms of abruptio placentae and pregnancy loss caused by motor vehicle crashes. The results were intended to be used to develop strategies to improve protection of the fetus in this setting.
Study design: Four integrated projects were conducted: (1) seated anthropometric measurements and belt fit determination during pregnancy, (2) development of new models of traumatic abruptio placentae, (3) investigations of crashes involving pregnant women, and (4) the development of the second-generation pregnant crash dummy from these data and others.
Results: Twenty-two different pregnant subjects in five different height groups underwent serial measurements of abdominal surface contours, seat belt fit, and distances between the subjects and various landmarks in the automobile interior with a laboratory-designed "automobile seat" (seating buck). The abdomen was significantly closer to the steering wheel in the shorter stature group than among the taller women. Beginning at approximately 20 weeks' gestation the fundus of the uterus was above the lower rim of the steering wheel. Lap belts fit properly over the anterior superior iliac spine throughout gestation, but the lap belt overlapped the uterus in the midsagittal plane. Two separate mechanisms for traumatic abruptio placentae were tested: shear failure and tensile failure. In the shear failure model large circumferential strains in the uterine wall induce a shear strain across the uteroplacental interface, and the model predicts placental separation at a mean circumferential strain of -58% +/- 8%. By means of finite-element modeling, it was demonstrated that tensile failure might also be a mechanism that causes abruptio placentae during rapid deceleration of the uterus. Crash investigations were performed in 43 cases involving pregnant women beyond 20 weeks' gestation. There were a total of 8 fetal losses and 8 major complications (fetal survival with abruptio placentae, direct fetal injury, or preterm delivery before 34 weeks' as a result of the accident). The best predictors of fetal loss or adverse outcome were impact severity and proper seat belt use. With these newly acquired data a second-generation crash dummy, known as the Maternal Anthropomorphic Measurement Apparatus version 2b (MAMA-2b), is being developed. It incorporates strain gauges in the fundal region of the fluid-filled uterus plus pressure transducers in both the anterior and posterior uterus. Criteria are being developed to associate the likelihood of abruptio placentae with measurements from these instruments that correspond to the two major hypothesized mechanisms of abruptio placentae.
Conclusion: An improved understanding of the elements of automobile crashes that cause fetal loss and other major pregnancy complications has been gained through this series of investigations.