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Recent Trends in Fetal and Infant Outcomes Following Post-term Pregnancies Volume 22, No. 1 - 2001
Abstract All births and infant deaths in 1985-87 and 1992-94 in Canada, except
in Ontario and Newfoundland, were analyzed to assess the potential impact
of the recent increased use of elective labour induction for post-term
pregnancies. Probabilistic linkage was carried out of infant death
records (Canadian Mortality Database) and respective birth registrations
(Canadian Birth Database) for the periods 1985-87 and 1992-94. The combined
fetal and infant mortality declined by 20-30% between 1985-87 and 1992-94
at each gestational week beginning at 37 weeks, with no increased reduction
among post-term pregnancies. Asphyxia-related fetal and infant deaths,
the most likely cause of death being preventable by labour induction for
post-term pregnancies, did not decrease among post-term pregnancies. On
the contrary, a substantial decrease of asphyxia-related deaths was observed
at 37 and 38 weeks over the same periods of time. Because fetal and infant
deaths are rare events and because the number of pregnancies passing
42 weeks of gestation decreased dramatically during 1992-94, statistically
unstable results may be inevitable in the comparison of mortality in this
group of pregnancies. Introduction Post-term pregnancies are pregnancies that reach at least 42 weeks of gestation.1 Perinatal mortality and the occurrence of various obstetric complications have been found to be higher in post-term than in term pregnancies.2,3 Two general management approaches have been developed to reduce the risk of these adverse outcomes: elective labour induction when the pregnancy reaches 41 or 42 weeks of gestation, or expectant management with frequent fetal monitoring and selective labour induction. The two approaches remain controversial.4-9 Proponents of elective labour induction cite evidence from randomized controlled trials showing that labour induction is associated with reduced perinatal mortality.4-6 Concerns have been raised, however, that results from tightly controlled trials may not be applicable in routine practice and that widespread implementation of routine induction for post-term pregnancies may lead to increased rates of cesarean section and other obstetric interventions.7-9 Despite these controversies, the use of elective labour induction for post-term pregnancies has increased dramatically in Canada since the early 1990s, mostly at 41weeks of gestation.6 During this period of time, to our knowledge, the only apparent difference in obstetric care between post-term and term pregnancies in Canada was a tendency to induce labour electively for the former, but not for the latter. A recent observational study suggests that the increased use of elective labour induction for post-term pregnancies has contributed to the decline in fetal deaths among these pregnancies in Canada.6 However, this study did not assess infant mortality or causes of fetal death. We hypothesized that if routine, elective labour induction in recent years has had an important impact on the fetal and infant outcomes of post-term pregnancies in the form of the reduction of fetal and infant mortality, of asphyxia-related deaths compared to other causes of death, and of perinatal compared to post-neonatal death, it would be more evident in post-term pregnancies than in term pregnancies. We carried out an epidemiologic study using linked birth and infant death files to test these hypotheses.
We used data for live births and stillbirths from Statistics Canada's Canadian Birth Database10 for the years 1985-1994 and data for fetal and infant deaths from the Canadian Mortality Database for the years 1985-1995. Fetal death is defined as stillbirth with birth weight >= 500 g or gestational age >= 20 weeks; neonatal death is defined as a live birth of an infant that died before the 28th full day of life; post-neonatal death is defined as a live birth of an infant that died between the 29th and the 364th full day of life; and infant death is defined as a live birth of an infant that died before the 364th full day of life. A probabilistic linkage was carried out using previously validated methods to link infant death records with respective birth registrations.11,12 Uncertain linkages were resolved after a manual examination of the relevant birth and death registration documents. Ontario births were excluded from the analysis because of documented problems with data quality.13 Newfoundland births were also excluded from the analysis of time trends, because data from this province were not available before 1991. Information in the linked files of live births and infant deaths was subjected to internal data quality checks, including procedures to exclude duplicate records. The linked birth and death information enabled the creation of birth cohorts with follow-up information on mortality in the first year after birth. Thus, infants born in 1985 were followed through 1986 to calculate infant mortality rates. Similarly, while the last birth cohort constructed was of live births in 1994, follow-up for infant death among newborns in this cohort extended through 1995. For the current study, only term and post-term births (i.e., those of 37 or more weeks of gestation) were included in the analysis. Because fetal and infant mortality decreased continuously during the 10-year period covered by the study and differences between successive years were small, we combined the data for 1985-1987 births and 1992-1994 births to enhance the statistical stability of the estimates. We calculated rates of fetal death, neonatal death, post-neonatal death, and overall fetal and infant death rates in each gestational week for the two study periods. Relative risks and 95% confidence intervals were used to compare mortality rates between the two study periods, with 1985-87 serving as the reference. We further analyzed fetal and infant mortality caused by asphyxia, on which labour induction probably has the largest impact. Only one underlying cause of death is recorded in Statistics Canada's Canadian Mortality Database, which is coded using the International Classification of Diseases, 9th Revision (ICD-9) classification system. We used a classification system adopted by the International Collaborative Effort on Perinatal and Infant Mortality14 to group codes with clinical conditions that may directly or indirectly lead to asphyxia-related death. These conditions include maternal death (ICD-9 761.6), malpresentation before labour (ICD-9 761.7), placenta previa or other placental abnormalities (ICD-9 762.0-762.2), prolapsed cord or other unspecified conditions of umbilical cord (ICD-9 762.4, 762.5, 762.6), breech delivery and extraction (ICD-9 763.0), disorders relating to long gestation and high birth weight (ICD-9 766), birth trauma (ICD-9 767), intrauterine hypoxia and birth asphyxia (ICD-9 768), meconium aspiration syndrome (ICD-9 770.1), subarachnoid hemorrhage (ICD-9 772.2), convulsions in newborn (ICD-9 779.0) and coma or other abnormal cerebral signs (ICD-9 779.2). We considered the potential impact of the reclassification of gestational age among post-term pregnancies, caused by more frequent labour induction at 41 or more weeks of gestation,6 on gestational age-specific mortality, and then carried out a parallel analysis combining all births at 41 or more weeks and compared the results with the main analysis using finer groupings of gestational age (37, 38, 39, 40, 41, 42 and >= 43 weeks of gestation).
The number of births and deaths are presented in Tables 1 and 2. Compared with 1985-87, births at 37, 38, 39 and 41 weeks of gestation increased as a proportion of total births in 1992-94, but decreased at 40 and (especially) 42 and >=43 weeks (Table 1). The results of the comparison of fetal and infant mortality rates between 1985-87 and 1992-94 are presented in Table 3. The fetal death rate at 37 weeks decreased by 28% from 1985-87 to 1992-94. The reduction in fetal death rates increased sequentially from 40 weeks onwards, being 12%, 15%, 20% and 35% among those at 40, 41, 42 and >= 43 weeks respectively. On the other hand, the decrease in neonatal mortality was greatest at 40 weeks (39% decrease in 1992-94 versus 1985-87; see Table 3). Compared with births at 40 weeks, the decreases were smaller at 37, 38, 39 and 41 weeks and there was no statistically significant reduction at 42 weeks (relative risk [RR] 0.94, 95% confidence interval [CI] 0.64-1.37). At >= 43 weeks, there was even a statistically nonsignificant increase. Post-neonatal mortality declined at every week of gestation in 1992-94 compared to 1985-87; this decrease was most evident at 41 or more weeks of gestation. The combined fetal and infant mortality rate had decreased by 20%-30% for every week of gestation in 1992-94 compared to 1985-87. The reduction at 41 or more weeks of gestation was slightly larger than the reduction at 37-40 weeks, mainly because of a larger reduction in post-neonatal mortality rates at these gestations. In general, fetal and infant mortality caused by asphyxia decreased in 1992-94 compared to 1985-87. However, the difference was statistically significant only for 37 and 38 weeks of gestation. Results obtained from analysis combining all births at 41 or more weeks (second row from bottom of Table 3) were generally consistent with those using finer categories of post-term pregnancies.
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Discussion Our study showed a moderate increase in the proportion of births at 41 weeks of gestation and substantial decreases in this proportion at 42 weeks and >= 43 weeks in 1992-94 compared to 1985-87. These changes are probably partly attributable to better dating of pregnancies with more frequent use of ultrasound early in pregnancy,15,16 although more frequent labour induction at >= 41 weeks has doubtless also played a role. Although we have no direct data on labour induction, a recent study showed a substantial rise in rates of labour induction at 41 weeks from the early 1990s in the majority of Canadian hospitals surveyed.6 The continued increase in inductions among post-term pregnancies after 1991 has probably been fueled, at least in part, by evidence from randomized controlled trials supporting this practice4,5 and by the increasing availability of intracervical and vaginal prostaglandin gels to assist with cervical ripening.6 During the same period of time, we observed a relatively larger (but statistically nonsignificant) reduction in fetal death among pregnancies at >= 41 weeks as compared with those at 40 or 39 weeks. This finding is consistent with a recent Canadian study.6 It is notable that the decrease in neonatal mortality at 41 weeks was not greater than at 40 weeks, and that no decrease occurred at 42 or >= 43 weeks; in fact at >= 43 weeks there was a statistically nonsignificant increase (Table 3). In general, larger reductions in fetal death have been accompanied by smaller reductions in neonatal mortality. For example, the decrease in fetal death was greater at 37 and 38 weeks than at 39 and 40 weeks, whereas the reverse was true for the decrease in neonatal mortality. This pattern of smaller reductions in neonatal mortality than in fetal mortality raises the possibility that more frequent use of medical interventions such as labour induction may have merely postponed some deaths. An alternative explanation for this phenomenon is that labour induction may be beneficial in certain circumstances but harmful in others. The reduction in post-neonatal mortality in 1992-94 versus 1985-87 was larger at >= 41 weeks than at 37 to 40 weeks (Table 3). This finding is in contrast to randomized controlled trials assessing the efficacy of labour induction for post-term pregnancies that have focused on perinatal mortality.4 It is possible that labour induction may have a larger impact on reducing post-neonatal mortality than it does on perinatal mortality. If this is the case, the assessment of the efficacy of labour induction for post-term pregnancies should be expanded to include the post-neonatal period. We hypothesized that labour induction for post-term pregnancies would have the largest impact on asphyxia-related fetal and infant mortality and morbidity. The reduction observed in fetal and infant mortality due to asphyxia-related conditions at >= 41 weeks, however, was not larger than at 40 weeks (Table 3). During the study period, the clinical definition for certain asphyxia-related conditions such as respiratory distress may have changed. However, our earlier study based on hospital discharge data found that the incidence of coded diagnoses such as respiratory distress and meconium aspiration syndrome was quite stable during the study period.17 Moreover, since we used a broad and inclusive definition of asphyxia-related deaths, such a shift in clinical definition probably had a limited impact on our study results. Our population-based results should reflect routine practice better than controlled trials. We realize the limitations inherent in any observational study based on administrative databases. Such data are prone to a certain degree of coding errors,18 which may be random or may contain systematic biases. The observational study design and the lack of information on induction in the data render any inference about the relation between labour induction and fetal and infant mortality necessarily indirect. Moreover, because fetal and infant deaths are rare events, and because the number of pregnancies passing 42 weeks decreased dramatically during 1992-1994, statistically unstable results were inevitable in the comparison of mortality in this group of pregnancies. Nonetheless, our findings are biologically plausible.Trials and guidelines vary widely in terms of gestational age for induction. In the Cochrane systematic review, trials demonstrated an effect of reducing perinatal mortality only for induction conducted after 42 weeks of gestation.4 The Society of Obstetricians and Gynecologists of Canada initially recommended that women who reach "41-42" weeks of gestation should be offered elective induction,19 and in practice the tendency has been to induce pregnancies closer to 41 rather than 42 weeks.6 The most difficult challenge in the management of post-term pregnancy may be related to determining the exact time that a given pregnancy becomes "post-term."9 It is thus difficult to establish a rigid and arbitrary cut-off point for induction. Acknowledgments We thank the Vital Statistics Registrars of the provinces and territories who gave us access to their data files. This study was conducted under the auspices of the Canadian Perinatal Surveillance System.
1. Bakketeig L, Bergjo P. Post-term pregnancy: magnitude of the problem. In: Chalmers I, Enkin M, Keirse MJNC, Editors. Effective care in pregnancy and childbirth. Oxford (UK): Oxford University Press; 1989. p. 765-75. 2. McClure Browne JC. Postmaturity. Am J Obstet Gynecol 1963;85:573-82. 3. Shime J, Gare DJ, Andrews J, Bertrand M, Salgado J, Whillans G. Prolonged pregnancy: surveillance of the fetus and the neonate and the course of labour and delivery. Am J Obstet Gynecol 1984;148:547-52. 4. Cowley P. Interventions for preventing or improving the outcome of delivery at or beyond term [Cochrane review]. In: The Cochrane Library; Issue 1, 1999. Oxford: Update Software. 5. Hannah ME, Hannah WJ, Hellman J, Hewson S, Milner R, Willan A and the Canadian Multicenter Post-term Pregnancy Trial Group: Induction of labour as compared with serial antenatal monitoring in post-term pregnancy. N Engl J Med 1992;326:1587-92. 6. Sue-A-Quan AK, Hannah ME, Cohen MM, Foster GA, Liston RM. Effect of labour induction on rates of stillbirth and cesarean section in post-term pregnancies. Can Med Assoc J 1999;160:1145-9. 7. Luther ER. Post-dates and induction: Where are we now? Reproductive Care Program of Nova Scotia Newsletter, June 1996. 8. Keirse MJNC. Post-term pregnancy: New lessons from an unresolved debate. Birth 1993;20:102-5. 9. Sanders N, Paterson C. Effect of gestational age on obstetric performance: when is "term" over? Lancet 1991;338:1190-2. 10. Fair ME, Cyr M. The Canadian birth database: a new research tool to study reproductive outcomes. Health Rep 1993;5:281-90. 11. Howe GR, Lindsay J. A generalized iterative record linkage computer system for use in medical follow-up studies. Computer Biomed Res 1981;14:327-40. 12. Smith ME, Silins J. Generalized iterative record linkage system. In: Proceedings of the American Statistical Association, Social Statistics Section, 1981:128-37. 13. Joseph KS, Kramer MS. Recent trends in infant mortality rates and proportions of low-birth-weight live births in Canada. Can Med Assoc J 1997;157:535-41. 14. Cole S, Hartford RB, Bergsjo P, McCarthy B. International Collaborative Effort (ICE) on birth weight, plurality, perinatal and infant mortality. Acta Obstet Gynecol Scand 1989;68:113-7. 15. Kramer MS, McLean FH, Boyd ME, Usher RH. The validity of gestational age estimation by menstrual dating in term, preterm and post-term gestations. JAMA 1988;260:3306-8. 16. Goldenberg RL, Davis RO, Cutter GR, Hoffman HJ, Brumfield CG, Foster JM. Prematurity, postdates, and growth retardation: the influence of use of ultrasonography on reported gestational age. Am J Obstet Gynecol 1989;160:462-70. 17. Wen SW, Liu S, Fowler D. Trends and variations in neonatal length of in-hospital stay in Canada. Can J Public Health 1998;89:115-9. 18. Huston P, Naylor CD. Health services research: Reporting on studies using secondary data sources. Can Med Assoc J 1996;155:1697-702. 19. SOGC Committee Opinion. Management of post-term pregnancy. April 1994:29.
Author References Shi Wu Wen, The Bureau of Reproductive and Child Health, Centre for Healthy
Human Development, Health Canada, Ottawa, Ontario; and Department of Obstetrics
and Gynecology and Clinical Epidemiology Program, University of Ottawa,
Ottawa, Ontario |
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