1 Harborview Injury Prevention and Research Center, Seattle, WA
2 Department of Epidemiology, School of Public Health and Community Medicine, University of Washington, Seattle, WA
3 Division of Public Health Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA
Correspondence to Dr. Melissa A. Schiff, Harborview Injury Prevention and Research Center, 325 Ninth Avenue, Box 359960, Seattle, WA 98104 (e-mail: mschiff{at}u.washington.edu).
Received for publication July 28, 2004. Accepted for publication October 26, 2004.
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ABSTRACT |
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accidents, traffic; hospitalization; pregnancy outcome; wounds and injuries
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INTRODUCTION |
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Research on adverse pregnancy outcomes following motor vehicle crashes during pregnancy has been limited. Wolf et al. (6) utilized a large, retrospective cohort study design to evaluate the effect of seat-belt use on pregnancy outcome following a motor vehicle crash. Compared with restrained pregnant women, unrestrained pregnant women had a 2.3-fold increased risk of delivering a low-birth-weight infant (<2,500 g). Using a case series, Pearlman et al. (7
) reported that 62 percent of improperly restrained women had adverse fetal outcomes compared with 27 percent of properly restrained women. Another case series of pregnant women in motor vehicle crashes documented that greater crash severity was associated with fetal death and that restraint use decreased the risk of adverse fetal outcomes (8
). Because adverse pregnancy outcomes occur in the general population of pregnant women as well as in the population of pregnant women involved in motor vehicle crashes, it is important to evaluate the excess risk associated with these crashes. Although a recent study of pregnant motor vehicle occupants found no increased risk of adverse pregnancy outcomes among those in crashes compared with pregnant women not involved in motor vehicle crashes (3
), the study did not evaluate outcomes stratified by injury severity. Our study evaluated the risk of adverse maternal and perinatal outcomes for pregnant women hospitalized following motor vehicle crashes, stratified by maternal injury severity.
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MATERIALS AND METHODS |
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Exposure classification
We used ICD-9-CM diagnosis codes to classify types of injuries from motor vehicle crashes, as follows: fractures, dislocations, sprains, and strains (codes 800849); intracranial injuries (codes 850854); internal injuries to the chest (codes 860862), abdomen (codes 863866, 868), and pelvis (code 867); open wounds (codes 870897); injury to blood vessels (codes 900904); superficial injuries, contusions, and crushing injuries (codes 910929); and nerve and spinal cord injuries (codes 950957). Women with multiple types of injuries were classified as having all applicable injury types.
We evaluated the severity of crash-related injuries by using ICDMAP-90 computer software (Tri-Analytics Inc., Bel Air, Maryland) to determine the Injury Severity Score (ISS) (9). The ICDMAP-90 software assigns the ISS based on ICD-9-CM diagnosis codes in the hospitalization data. Of the 625 pregnant women involved in a motor vehicle crash, the ICDMAP-90 program was unable to assign an ISS for 43 because their injury diagnosis was not coded to a specific body region. These 43 women were excluded from the analyses of pregnancy outcomes. Of the remaining 582 pregnant women, 84 were classified as having severe injuries with an ISS of
9, and 309 were classified as having nonsevere injuries with an ISS of 18. An ISS cutpoint of 9 was used because prior studies of pregnant trauma patients reported that this score predicted fetal death following injury during pregnancy (10
, 11
). The remaining 189 pregnant women experienced trauma (presence of an E code) but no injury (absence of an ICD-9-CM injury code) and were assigned an ISS of 0.
Outcome classification
To evaluate maternal and perinatal outcomes, we used the Washington State Birth Event Records Database, which contains the maternal and infant ICD-9-CM diagnosis and procedure codes for the delivery hospitalization, linked to the birth certificate or fetal death record. A prior study evaluating the linkage procedure for creating the Birth Event Records Database file reported that the linkage was 96 percent complete (12). Since 1980, the Washington State birth and fetal death certificates have included a checklist for obstetric procedures, method of delivery, complications of labor and delivery, and abnormal conditions of the newborn. Maternal outcomes investigated included the following: preterm labor at less than 37 weeks' gestation (ICD-9-CM codes 644.0, 644.2 or use of tocolysis noted on the birth or fetal death certificate); premature rupture of membranes 24 hours or more prior to the onset of labor (ICD-9-CM code 658.2); placental abruption (ICD-9-CM code 641.2 or as listed on the birth or fetal death certificate); induction of labor (ICD-9-CM diagnosis codes 659.0, 659.1; procedure codes 73.1, 73.4, 96.49; or as listed on the birth or fetal death certificate); cesarean delivery (ICD-9-CM codes 7474.2 or as listed on the birth or fetal death certificate); and maternal death (death of the mother during the hospitalization using the discharge status in the Comprehensive Hospital Abstract Recording System record).
Perinatal outcomes investigated included gestational age of less than 37 weeks (as recorded on the birth or fetal death certificate); low birth weight (<2,500 g listed on the birth or fetal death certificate); moderate-to-heavy meconium at delivery (noted on the birth or fetal death certificate); fetal distress (ICD-9-CM codes 656.3, 768.2768.4 or as listed on the birth or fetal death certificate); mild-to-severe birth asphyxia (ICD-9-CM codes 768.5768.9); and respiratory distress syndrome (ICD-9-CM code 769 or as listed on the birth certificate). We used an algorithm by Alexander et al. (13) to define small for gestational age as infant birth weight below the 10th percentile for a given gestational age, stratified on sex and race/ethnicity. We derived gestational age at the time of the motor vehicle crash by subtracting the number of weeks between the crash hospitalization and the birth/fetal death. We defined fetal death as an intrauterine death identified by a fetal death certificate.
Data analysis
We compared the demographic and obstetric characteristics of women hospitalized for a motor vehicle crash during pregnancy (n = 625) with those of women with no history of hospitalization for a motor vehicle crash during pregnancy (n = 17,274). We assessed proportions of types of injuries (using ICD-9-CM diagnosis codes) among the pregnant women in a motor vehicle crash who experienced severe and nonsevere injuries. We used Poisson regression analysis to calculate relative risks and 95 percent confidence intervals for associations between ISS categories and maternal and perinatal outcomes among those for whom data on pregnancy outcomes and confounding variables were complete. In our initial analyses, we built a regression model for each pregnancy outcome comparing all pregnant women involved in crashes with pregnant women who were not in crashes. Because we wanted to assess the differences in pregnancy outcomes stratified by ISS, we built additional regression models for each pregnancy outcome comparing pregnant women sustaining severe motor vehicle crash injuries (ISS 9), pregnant women experiencing nonsevere crash injuries (ISS 18), and pregnant women receiving no injuries in crashes (ISS 0) with pregnant women not involved in crashes.
We performed subanalyses of adverse pregnancy outcomes for the pregnant women in crashes who had no injuries (ISS 0), stratified by those who were hospitalized and subsequently discharged without delivering and those who delivered at their crash hospitalization. We assessed for possible interactions between ISS and gestational age and found no significant interaction. We also assessed for possible confounding by age, race/ethnicity, marital status, education, income, gravidity, parity, prenatal smoking, type of prenatal care insurance, and trimester that prenatal care was initiated. Relative risks were adjusted for age and prenatal smoking because these factors were associated with both motor vehicle crash exposure and maternal and perinatal outcomes and changed one or more of the relative risks by at least 10 percent. For selected maternal and perinatal outcomes for which there were fewer than five observations among the injured women, we present unadjusted relative risks in this paper. All statistical analyses were performed by using Stata version 8.0 software (Stata Corporation, College Station, Texas).
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RESULTS |
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Stratifying our analysis by ISS, we found that severely injured pregnant women (ISS 9) had a 1.6-fold increased risk of cesarean delivery and a ninefold increased risk of placental abruption compared with pregnant women not involved in motor vehicle crashes (table 3). Two (2.4 percent) of the severely injured pregnant women died in crashes compared with two (0.01 percent) of the pregnant women not in crashes. The risk of adverse perinatal outcomes was also increased for the severely injured pregnant women, with a twofold increased risk of fetal distress, a fourfold increased risk of infant respiratory distress syndrome, and a ninefold increased risk of fetal death. Non-severely injured pregnant women (ISS 18) were at increased risk of preterm labor, placental abruption, and cesarean delivery compared with pregnant women not involved in a motor vehicle crash.
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DISCUSSION |
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Although prior studies (14, 15
) have noted that crash severity is related to adverse outcomes, with more-severe crashes resulting in more-severe injuries and adverse pregnancy outcomes, less-severe crashes also have the potential for adverse outcomes. Among pregnant women involved in crashes who sustained nonsevere injuries, our findings of increased risks of preterm labor, placental abruption, and cesarean delivery confirm those from earlier non-population-based studies. In the case series by Klinich et al. (8
), 41 of 120 pregnant women involved in crashes experienced placental abruption with only minor maternal injuries. Several other studies of noncatastrophic trauma during pregnancy have reported preterm labor (16
, 17
), placental abruption (16
, 17
), and cesarean section (18
) in case series of non-severely injured pregnant women. These studies of noncatastrophic injuries could not determine the magnitude of any potential association between injury and outcomes because of a lack of comparison groups of uninjured pregnant women. Our findings of adverse outcomes among non-severely injured pregnant women involved in crashes indicate that relatively minor trauma can have a substantial impact on pregnancy outcomes. Our findings also illustrate that the ISS in pregnancy is a poor predictor of adverse pregnancy outcomes, as documented in a prior study of the use of ISS scores to predict adverse pregnancy outcomes (19
). It may be useful to develop an injury severity scoring system specific to pregnancy to evaluate patients who experience trauma during pregnancy.
We found that, compared with pregnant women not involved in crashes, pregnant women hospitalized with no reported injuries after motor vehicle crashes were at increased risk of adverse pregnancy outcomes including preterm labor, placental abruption, and infant respiratory distress syndrome. The increased risk of preterm labor may be explained by the forces of the crash stimulating uterine contractions, although the biomechanics of crash forces on the contractility of the pregnant uterus have not been studied to our knowledge. Placental abruption in uninjured pregnant women involved in crashes was also reported by Klinich et al. (8) in a large case series of 120 pregnant women in crashes, 10 of whom experienced placental abruption with no maternal injuries. Biomechanical studies of uterine and placental tissue and finite-element modeling of the pregnant uterus have documented that the shear forces of the crash impact can cause the placenta to separate from the uterus (7
). Another study using finite-element modeling of the pregnant uterus reported that contact with the steering wheel resulted in strain at the uteroplacental interface sufficient to cause placental abruption (20
). Although we found no fetal deaths among the uninjured pregnant women involved in crashes, a case series of pregnant trauma patients by Poole et al. (21
) included five uninjured pregnant women who experienced a fetal death. Our findings of adverse outcomes for pregnant women in crashes with no documented injuries indicate that crashes resulting in no maternal injuries can increase the risk of adverse pregnancy outcomes. Since our study included only those pregnant women who were hospitalized following motor vehicle crashes, there was the potential for selection bias; those women who had adverse obstetric or fetal signs such as contractions, uterine tenderness, vaginal bleeding, or fetal distress may have been more likely to be hospitalized despite having no crash-related injuries. Because we used existing databases and did not evaluate medical records, it was difficult to determine the exact chain of events following a motor vehicle crash.
Our study had several limitations. The most important was that our data were limited to pregnant women who were hospitalized following a crash. By limiting our study to these women, we likely included pregnant women who sustained more-severe injuries and those who were at increased risk of adverse outcomes and excluded pregnant women who experienced more minor injuries and those not at increased risk of adverse outcomes. These exclusions could possibly result in overestimation of the magnitude of risk of adverse pregnancy outcomes following a motor vehicle crash. In addition, our finding that over half of the pregnant women were in their third trimester of pregnancy is also likely explained by the selection of hospitalized subjects. A significant proportion of women in a crash during their third trimester would seek care at a hospital for fetal assessment. Women in their first trimester and the early part of their second trimester would not require fetal monitoring because the fetus would be previable. A prior study of pregnant women in crashes that used police crash reports rather than hospitalizations to identify subjects noted a more uniform distribution of crashes by trimester (3).
Since the ISS was initially developed to predict survival following motor vehicle crashes in a nonpregnant population (9), and a prior study has shown that the ISS is a poor predictor of adverse outcomes among injured pregnant women (19
), its use in our pregnant cohort may have resulted in misclassification of the severity of injury or the impact of the injury on the pregnancy. Despite these limitations, we used the ISS to classify injury severity because other studies have used it as a standardized measurement in pregnant as well as nonpregnant populations and a pregnancy-specific injury scoring system is not available. Our results may have been biased by differential ascertainment of maternal and infant outcomes. Increased awareness of possible adverse outcomes following a crash may have resulted in differential reporting of certain subjective diagnoses (e.g., placental abruption, fetal distress, and infant respiratory distress syndrome) on the birth and fetal death certificates for the pregnant women in crashes compared with the pregnant women not involved in a crash. The more-objective diagnoses we examined (gestational age, birth weight, and fetal death) were not subject to this potential bias.
Limitations of coding and data entry in the Comprehensive Hospital Abstract Recording System and on the birth and fetal death certificates may have limited our ability to accurately ascertain demographic, exposure, and outcome information. Studies have documented that birth weight, race, age, and number of prior livebirths are recorded on birth and fetal death certificates with nearly complete accuracy (22), but prenatal and intrapartum complications, delivery method, and obstetric procedures are recorded less accurately (23
, 24
). Prior studies have also evaluated the accuracy of ICD-9-CM coding for selected obstetric diagnosis and procedure codes comparing discharge databases with medical records and have noted good-to-excellent accuracy (25
, 26
). We attempted to minimize inaccuracies by using multiple sources to identify specific maternal and perinatal outcomes when possible. Nondifferential misclassification of outcomes would have resulted in spurious attenuation of our risk estimates. We were also unable to evaluate pregnancy outcomes prior to 20 weeks' gestation because birth and fetal death certificates are used for pregnancies of more than 20 weeks only. Although it has been well documented that seat-belt use during pregnancy decreases the risk of adverse pregnancy outcomes (6
, 27
), we did not have information on seat-belt use or airbag deployment in our pregnant cohort.
In conclusion, we found that pregnant women hospitalized following motor vehicle crashes are at increased risk of adverse pregnancy outcomes, regardless of the presence of documented injuries or the severity of the injury. In light of the increased risk of adverse maternal and perinatal outcomes resulting from trauma due to crashes without injuries, careful maternal and fetal monitoring following a crash is warranted. Future studies evaluating police crash data in conjunction with birth and fetal death certificates and hospitalization data may provide additional insight into how specific types of crashes and crash forces play a role in adverse pregnancy outcomes. Additional biomechanical and clinical research will also provide much needed information on the effects of motor vehicle crashes during pregnancy.
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ACKNOWLEDGMENTS |
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The authors thank William O'Brien for supplying the data analysis files and Chris Mack at the Harborview Injury Prevention and Research Center for providing the injury severity scores by using the ICDMAP-90 software.
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References |
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