An International Contrast of Rates of Placental Abruption: An Age-Period-Cohort Analysis
An International Contrast of Rates of Placental Abruption: An Age-Period-Cohort Analysis
Cande V. Ananth 0
Katherine M. Keyes 0
Ava Hamilton 0
Mika Gissler 0
Chunsen Wu 0
Shiliang Liu 0
Miguel Angel Luque-Fernandez 0
Rolv Skjaerven 0
Michelle A. Williams 0
Minna Tikkanen 0
Sven Cnattingius 0
Chang-Qing Gao, Central South
0 1 Department of Obstetrics and Gynecology, College of Physicians and Surgeons, Columbia University , New York , New York, United States of America, 2 Department of Epidemiology, Mailman School of Public Health, Columbia University , New York , New York, United States of America, 3 THL National Institute for Health and Welfare , Helsinki , Finland and NHV Nordic School of Public Health , Gothenburg , Sweden , 4 Section for Epidemiology, Department of Public Health, Aarhus University , Aarhus , Denmark , 5 Health Surveillance and Epidemiology Division, Centre for Chronic Disease Prevention, Public Health Agency of Canada , Ottawa , Canada , 6 Department of Epidemiology, School of Public Health, Harvard University , Boston , Massachusetts, United States of America, 7 Department of Global Public Health and Primary Care, University of Bergen , Bergen , Norway , 8 Medical Birth Registry of Norway, Norwegian Institute of Public Health , Bergen , Norway , 9 Department of Obstetrics and Gynecology, University Central Hospital , Helsinki , Finland , 10 Clinical Epidemiology Unit, Department of Medicine Solna , Karolinska Institutet, Stockholm , Sweden
Data Availability Statement: United States: The
completely de-identified National Hospital Discharge
Survey (NHDS) data in the United States is publically
available at: http://www.cdc.gov/nchs/nhds.htm
Canada: Canadian hospitalization records data
cannot be released unless the request is
accompanied by an invitation for collaboration with
staff at the Public Health Agency of Canada (PHAC).
Since the data cannot be released to collaborators
outside of PHAC, all data analyses will be undertaken
by the PHAC staff. For further details, please contact
Shiliang Liu, MB, PhD at < Shiliang.Liu@phac-aspc.
gc.ca>. Sweden: Data Availability: This national
Although rare, placental abruption is implicated in disproportionately high rates of perinatal
morbidity and mortality. Understanding geographic and temporal variations may provide
insights into possible amenable factors of abruption. We examined abruption frequencies by
maternal age, delivery year, and maternal birth cohorts over three decades across seven
6,151,508; 199912) were analyzed. Abruption diagnosis was based on ICD coding. Rates
were modeled using Poisson regression within the framework of an age-period-cohort
analysis, and multi-level models to examine the contribution of smoking in four countries.
Abruption rates varied across the seven countries (310 per 1000), Maternal age showed a
consistent J-shaped pattern with increased rates at the extremes of the age distribution. In
comparison to births in 2000, births after 2000 in European countries had lower abruption
cohort study was based on data from the Swedish
Medical Birth Register (MBR). According to Swedish
law, the authors are not able to share the register
data used in this study with other researchers. For
further information, please see http://www.
default.aspx (in Swedish language) Norway:
Norwegian Medical Birth Registry data is available to
the public, and requests for data can be made
through an electronic application. For further details,
please refer to http://www.fhi.no/eway/default.aspx?
6898:0:25,7913:1:0:0:::0:0 Denmark: Data
Availability: This national cohort study was based on
data from the Danish Medical Birth Register (MBR).
According to Danish law and Data Protect Agency,
the authors are not able to share the register data
used in this study with other researchers. For further
information, please see http://www.ssi.dk/
(in Danish language). Finland: For data access,
please refer to http://www.thl.fi/en/web/thlfi-en/
http://www.thl.fi/en/web/thlfi-en/statistics/statistics-bytopic/sexual-and-reproductive-health/parturientsdeliveries-and-births/perinatal-statistics-parturientsdelivers-and-newborns Contact: Mika Gissler, THL
National Institute for Health and Welfare, PO BOX 30,
00271 Helsinki, Finland, Spain:
estadisticas/cmbdhome.htm For more information
and documentation and submission of data request,
please contact the Health Information Institute:
Funding: This is an unfunded project. Dr. Keyes is
partially supported by a grant (K01-AA021511) from
the National Institutes of Health. None of the authors
received specific funding for this work.
Competing Interests: The authors have declared
that no competing interests exist.
rates; in the US there was an increase in rate up to 2000 and a plateau thereafter. No birth
cohort effects were evident. Changes in smoking prevalence partially explained the period
effect in the US (P = 0.01) and Sweden (P<0.01).
There is a strong maternal age effect on abruption. While the abruption rate has plateaued
since 2000 in the US, all other countries show declining rates. These findings suggest
considerable variation in abruption frequencies across countries; differences in the distribution
of risk factors, especially smoking, may help guide policy to reduce abruption rates.
Roughly a third of all pregnancies in high-income countries are diagnosed with some
medical or obstetrical complication, contributing to the substantial burden of maternal and
perinatal morbidity and mortality. Placental abruption, one of the severest of all obstetrical
complications, is a condition when the placenta detaches prematurely . The complication is
associated with disproportionately high rates of perinatal mortality and morbidity and
neurodevelopmental deficits in children later in life . Women diagnosed with abruption suffer 3
to 4-fold increased risk of premature cardiovascular mortality and, morbidity [6,7].
The prevalence of abruption in European countries is 36 per 1000 pregnancies ,
whereas the corresponding data in North America is two-fold higher (712 per 1000
pregnancies). Furthermore, recent European studies have documented a temporal decline in abruption
rates [9,11], but data from the US and, Canada have shown increased rates . The change
in the prevalence rate of abruption and the contrasting geographical heterogeneity in both
rates and trends suggest that population-based risk factors for abruption may contribute to
differences in rates. An international contrast of abruption rates using an age-period-cohort
(APC) analysis may provide insights toward an understanding of how and why abruption
trends change over time.
There is substantial empirical evidence that supports a potential contribution of age, period,
as well as cohort effects to abruption rates. Maternal age shows a J-shaped distribution of risk
for abruption, with young (<20 years) and advanced maternal age ( 35 years) at increased
risk . By period and cohort, temporal trends in risk factors for abruption such as cigarette
smoking [16,17] may be contributing to the observed temporal variation. Smoking during
pregnancy is a strong and consistently observed risk factor for abruption [4,12,1721]. With
the exception of Finland where the prevalence of smoking during pregnancy has remained
stable at 15%, smoking has generally decreased in the US, Canada, Sweden, Denmark and Spain
in the last two decades .
In a large, population-based study of births in the US, Canada, Sweden, Norway, Denmark,
Finland and Spain, we examine abruption rates across three decades with regard to maternal
age, time of occurrence of abruption and maternal year of birth (cohort). We hypothesize a
decline in birth cohort effects on abruption, largely driven by the declining prevalence of
We implemented a population-based, cohort analysis of births over three decades in the United
States, Canada, Sweden, Norway, Denmark, Finland and Spain. The study was restricted to
women delivering a singleton live birth or stillbirth (with the exception of Denmark that
included only live births).
Since data for all countries are de-identified, they do not qualify as human subjects research;
therefore, no ethics approval from an Institutional Review Board was sought for this study.
United States births, 19792010
Data for US births were derived from the 19792010 National Hospital Discharge Survey
(NHDS) datasets, assembled by the US Centers for Disease Control and Prevention. These data
are based on hospitalizations from short-stay (<30 days) hospitals, and non-federal general
and specialty hospitals. The data included in the survey are representative of sample of hospital
discharges in the 50 states and the District of Columbia.
Between 1979 and 1987, the survey used a two-stage stratified sampling design; since 1988
the survey used a modified three-stage design [26,27]. The number of hospitals surveyed each
year ranged between 400 and 558 and included approximately 181,000 to over 300,000 hospital
discharges each year. Beginning in 2008, the number of hospitals chosen for the survey was
reduced by half (roughly 200 to 300 hospitals). When the sampling weights are applied, these
data represent all in-patient hospitalizations in the US. All diagnoses and procedures were
coded based on the International Classification of Disease (ICD), 9th revision, Clinical
Canada births, 19812010
Data on Canadian births were abstracted from the Canadian Institute for Health Informations
hospital Discharge Abstract Database (DAD). The DAD contains data on every hospitalization,
including demographic and residence information, length of stay, diagnoses and procedures
performed during the hospitalization. Up to 200001 all diagnoses were coded according to
ICD-9-CM, and procedures were coded according to the Canadian Classification of Diagnostic,
Therapeutic, and Surgical Procedures. Since 2001, the ICD-10 and the Canadian Classification
of Health Interventions for procedures were adopted by the participating hospitals. The DAD
includes data on deliveries of all live births and stillbirths. Only hospitalizations occurring in
the provinces of Prince Edward Island, Ontario, Saskatchewan, and British Columbia were
complete in the DAD over the entire period of study (accounting for over 50% of the whole
Canadian population), and were analyzed in this study [28,29].
Sweden births, 19782010
Data were derived from the nation-wide Swedish Medical Birth Register (SMBR), maintained
by the National Board of Health and Welfare. Data in the SMBR is collected through
standardized prenatal, obstetric and neonatal records, which are forwarded to the registry after delivery.
The SMBR includes information on more than 98% of births in Sweden . All diagnoses and
procedures were coded based on the Swedish versions of the ICD-8 (19781986), ICD-9
(19871996) and ICD-10 (1997 and later) revisions.
Norway births, 19782009
Based on compulsory notification, the Medical Birth Registry of Norway (MBRN) has since
1967 recorded medical data on all births in Norway with a gestational age of at least 16 weeks,
including data on maternal disease and complications during pregnancy. The attending
midwife and doctor using a standardized notification form, either as free text or by predefined
variables or check boxes, register data. Free text is coded at the MBRN using ICD-8 revision for
years 196798 and ICD-10 since 1999. MBRN contains all births in Norway for all years since
Denmark births, 19782008
Data for Danish births were derived from the Danish Birth Register and the Danish Hospital
Register. We identified all live-born infants between 1978 and 2008 from the Danish Medical
Birth Register, which includes all births in Denmark since 1973 . All live-born infants and
new residents in Denmark are assigned a unique civil registration number, which were used to
link information from several national registries at the individual level. Data on abruption was
extracted from the Danish National Hospital Register that holds nationwide data on all
admissions to any Danish hospital since 1977 and on all outpatient visits since 1995. The discharge
diagnoses are based on the Danish version of the ICD-8 (19771993), and ICD-10 (1994
Finland births, 19872010
The linked Finnish Hospital Discharge Register (FHDR) and the Medical Birth Register
(FMBR) were used to identify women with and without an abruption diagnosis based on the
ICD-9 (19871995) or ICD-10 (19962010) codes in the FHDR and a check box in the FMBR
(since October 1990). The FHDR and FMBR were linked using womens unique encrypted
identification number. The FMBR (since 1987) collects baseline data on health care and
interventions women receive during pregnancy and delivery and newborn outcome during the first
7 days (<0.1% of newborns are missing in the FMBR). For all variables used in this study, the
data have been validated and correspond well with information available in hospital records.
The HDR collects information on inpatient episodes (since 1969), outpatient surgical
procedures (since 1994), and outpatient visits in public hospitals (since 1998) [33,34].
Spain births, 19992012
Data were derived from the National Institute of Statistics (NIS) in Spain. We used the
vitalstatistics database, for which the official data source is the standardized birth-registration form.
This database contains information on all the births in Spain (http://www.ine.es). Data
regarding diagnoses were derived from the Department of Health. We used the Spanish hospitals
discharge database with codes based on the Spanish versions of the ICD-9-CM (19992012).
These data files contain data on all public hospitals in Spain (www.msssi.gob.es/estadEstudios/
estadisticas/cmbd/informes/notasMetodologicas.htm; accessed 1 April 2014).
Classification and diagnosis of placental abruption
There are no standard or universally acceptable diagnostic criteria for placental abruption. The
clinical criteria for an abruption diagnosis includes painful bleeding accompanied with tetanic
uterine contractions or fetal heart rate decelerations; or evidence of blood clots embedded on
the placental surface; or evidence of retroplacental bleeding behind the placental margin .
The ICD codes that were used to extract data on abruption included ICD-8 6321 and 6514;
ICD-9 6412A and 6412B; and ICD-10 O450, O458 and O459.
Age, period, and cohort (APC) effects were modeled using the classical approach for tabular
data developed by Clayton and Shifflers [36,37]. Age, period, and cohort are linearly dependent
variables (Cohort = Period-Age), thus modeling strategies require typically focus on capturing
non-linear variation in tabular rates over time. Given the strong age effects associated with
abruption, we first estimate abruption rates as a function of age. Then, we include a parameter
for the overall linear change in abruption rates over time (termed drift) that is a sum of both
period and cohort effects but not uniquely attributable to either. Finally, unique coefficients for
period and cohort effects (termed curvatures) are obtained and interpreted as the
secondorder derivatives of the drift parameter.
The best fitting model of the data is selected by assessing the change in model fit (assessed
by the likelihood-based deviance statistics and penalizing additional degrees of freedom) over
successive models, beginning with an age parameter only, then age + drift, then the curvature
of period given age and drift, etc. The modeling strategy was determined a priori, and was
implemented with the function apc.fit in the Epi package  in the R software (version
R.3.0.3; R Foundation for Statistical Computing, Vienna, Austria). Data from each country was
Effects of changes in population rates of cigarette smoking. While the above strategy
allows for rigorous estimation of APC effects, these models do not allow for the incorporation of
covariates, such as smoking, that may explain the observed APC effects in abruption. Therefore
we fit multilevel models that allowed for covariates to predict changes in the period and cohort
effect estimates of abruption over time . Period and cohort were considered cross-classified
random effects, and the aggregate smoking prevalence by country was tested to determine
whether it explained between-group variance in period and cohort parameters.
The annual prevalence of smoking during pregnancy (yes/no) was available from five
countries: US, Canada, Sweden, Norway and Finland. Data on smoking among US women (aged
15 years) giving birth each year was abstracted from the vital statistics data since 1989 (the
year when smoking data were ascertained on birth certificates). Smoking data in Canada since
1994 were obtained from the National Population Health Survey and the Canadian
Community Health Survey conducted by Statistics Canada . Data on smoking in Sweden, Norway
and Finland were ascertained at the first prenatal visit.
In the US, smoking decreased between 1989 (19.5%) and 2010 (9.2%); in Canada, from
35.5% in 1994 to 21.3% in 2009; in Sweden, from 30.9% in 1982 to 6.5% in 2010; and in Norway
from 18.9% in 1999 to 7.5% in 2009. In Finland, the smoking prevalence remained constant
between 1987 (15.0%) and 2010 (15.2%).
Analyses stratified based on maternal race. Given the well-known race disparity in
abruption in the US , we examined if APC effects on abruption rates differed based on
maternal race in the US. This sub-analysis was restricted to white (n = 468,257) and black
(n = 119,466) women.
Fig 1. Temporal changes in the prevalence rates of placental abruption between 1978 and 2012 in the US, Canada, Sweden, Norway, Denmark,
Finland and Spain.
The rate of abruption, across all years, was highest in the US (ranging from 7.4 in 1980 to 11.9
per 1000 in 2007) and lowest in Finland (ranging from 4.8 in 1994 to 3.3 per 1000 in 2009).
The rates of abruption in all European countries showed a temporal decline, and the most
dramatic decline was seen in Denmark (Fig 1). In North America, abruption rates increased in
Canada while the rate in the US fluctuated over time, but overall there was an increasing trend
over time also in the US.
Fig 2 shows the prevalence of abruption by maternal age, with each line reflecting a birth
cohort. For ease of visual interpretation, we present every fifth cohort, and increasing darkness of
the line represents increasingly younger cohorts. In all countries, there is a J-shaped
Fig 2. Rates of placental abruption in relation to maternal age within strata of maternal birth cohorts in the US, Canada, Sweden, Norway,
Denmark, Finland and Spain.
distribution, with the highest rates among the youngest and oldest mothers. This shape is less
pronounced in the US, wherein there is more overall variability in the rates. Further, these
graphs reveal the appearance of cohort effects in Canada, Sweden, Norway, Denmark, and
Spain, as the rates across age increase with increasingly younger cohorts. There is little evidence
of cohort effects in Finland and in the US.
Trends in placental abruption by risk factors
In Table 1 we present abruption rates in two years, 1981 (1987 in Finland) and 2008, by
maternal age. Among women <20 year old, across all countries except Canada and Sweden, the rate
decreased between the two periods; the largest relative decline in rate was in Denmark, and the
smallest in Spain. Between 1981 and 2008, abruption rates among women <20 years old
increased in both Canada and Sweden. There was a concomitant increase in abruption rates
among women aged 35 years in the US, Canada, and Finland, and a decline in Sweden,
Norway, Denmark and Spain.
Age, period, and cohort effects in placental abruption
APC model fit statistics are shown in Table 2. For all countries except Finland, a model with
age, drift, as well as period and cohort curvatures provided the best fit to the data. In Finland,
there was no significant change in model fit for any model, indicating that rates are best
described by a single age parameter.
Fig 3 shows the estimates of age, period, and cohort effects on abruption. The age-specific
rates are plotted on the left axis, and period and cohort estimates are plotted on the rate ratio
scale (right axis), with 2000 as the reference group for period and 1975 as the reference for
Similar to descriptive analysis, maternal age demonstrates a J-shaped relationship with
abruption. Of note is that while the overall abruption rate in Denmark (averaged over 1978
and 2008) was 8 per 1000, the rates displayed in Fig 3 indicate that the abruption rate was, on
average, 5 per 1000. This is likely consequence of strong confounding due to birth cohorts. The
Prevalence rate of placental abruption (per 1000 births)
Estimates are for 1987
Estimates for 1999. Change is estimated as the difference in abruption rates between the two periods relative to the rate in the earlier year
Change in deviance (change in degrees-of-freedom)
separation of a birth cohort effect from age and period shows an absence of cohort effect on
abruption rates. In all seven countries, there was a slight temporal increase in the rate of
abruption in the 1980s, and either a flattening of the rate (in the US) or a temporal decline, especially
among the European countries.
Changes in smoking as a potential explanatory factor. Using multi-level random effects
models, we estimated whether changes in the prevalence of smoking over time explain variance
in time-trends in abruption. We found that smoking prevalence significantly explained, at least
in part, the between-period variation in abruption in the US ( = 0.05, 95% confidence interval
0.03, 0.10 and in Sweden ( = 0.16, 95% confidence interval 0.09, 0.31), and coefficients
indicated that smoking rates are positively associated with period effects, suggesting higher rates of
abruption in time periods in which there is a higher prevalence of smoking (S1 Fig). Smoking
was not associated with between-group variance in Canada, and multi-level models for Norway
and Finland did not converge.
APC models were also estimated by race in the US; results were consistent with the overall
analysis for US (S2 Fig). There is a J-shaped relation between age and abruption, and no
evidence of a decline in abruption rates by period similar to what was demonstrated in European
countries among either White or Black mothers in the US.
The central goal of this international comparison is to evaluate the interactions of maternal
age, period and birth cohorts on temporal changes in the rates of abruption. Unlike the US, all
other countries show a clear decrease in abruption rates since 2000 or earlier, and in the US the
rates have plateaued since 2000. These findings suggest that population rates of abruption are
country specific and may be related to differences in the distribution of risk factors across
countries. There is a strong and consistent maternal age effect on abruption. We document
that changes in smoking prevalence are associated with changes in abruption rates in the US
Limitations of the data
There are a few limitations to this international comparison of abruption rates. Importantly,
selective fertility the necessity of couples to replace a pregnancy loss [42,43] may have
influenced these APC effects on abruption to some extent. Women with abruption that result in a
Fig 3. Age-period-cohort effects of placental abruption in the United States, Canada, Sweden,
Norway, Denmark, Finland, and Spain. Abruption rates (per 1000) based on maternal age are shown on
the left axis, and the rate ratio with 95% confidence interval for placental abruption in relation to maternal birth
cohort (year 1975 as the reference) and period (year 2000 as the reference) are shown on the right axis.
perinatal death are likely to replace the loss more quickly than women without any losses,
leading to short interval between pregnancies (resulting in folate depletion). This, in turn, may
increase the risk of abruption. Second, we were unable to account for repeat pregnancies within
women and the possibility of increased recurrence of abruption across successive pregnancies
[10,44,45]. A third limitation is the effect of risk factors that may account for the abruption
trends, notably maternal behavioral and sociodemographic factors. However, adjustments for
these factors is unlikely to modify the associations because the prevalence of sociodemographic
factors are unlikely to exhibit similar trends over time as is seen in abruption rates. Fourth, a
secular increase in the use of ultrasound to diagnose abruptions may have influenced the
Misclassification of abruption may have affected our findings to some extent and such
misclassification proportions probably vary across countries. In the US, for instance, the validity of
abruption diagnosis in the NHDS data contrasted against medical chart abstraction was fairly
high (sensitivity and positive predictive values of 89% each) . In Finland, the validity in the
FMBR and FHDR is high (97.8% agreement for diagnoses related to pregnancy, childbirth and
puerperium). The possibility of coding differences across ICD classification between ICD-8
and ICD-10 may have had some impact on our findings. The APC modeling of abruption rates
can also be subjected to residual confounding due to both measured and unmeasured factors.
While some potential explanatory variables are common across all countries (e.g., maternal
smoking), many are not (e.g., African-American race in the US where the abruption risk is
1.8-fold higher than non-Hispanic White women). Therefore, our approach was to focus on
the international comparison that is population-based; we only adjusted for changes in the
prevalence of smoking. Moreover, our adjustment for maternal smoking, at least in the US, was
based on smoking prevalence rates by year. Since patient-level smoking data were unavailable
(in the US), we cannot estimate the rate of abruption among women who smoke during
pregnancy versus those who do not. However, our goal in the sensitivity analysis was to estimate
the extent to which the overall population trends in smoking are associated with population
trends in abruption, rather than to estimate individual-level risks of abruption associated with
Appropriate APC modeling is an active area of methodological research. Every model is
subject to the limitations of the assumptions of the technique. In our technique, we assume
that age, period, and cohort have a linear effect on the outcome that is unobservable, and thus
estimate deviations from linearity. We also triangulate results across two modeling strategies,
including estimating drift and curvatures through Poisson regression as well as cross-classified
random effects models. We do not account for non-linear associations among the variables,
such as age by cohort or age by period interactions. However, the graphical data did not suggest
that such interactions are likely to play a critical role in these estimates.
Strengths of the study
With the exception of births in the US, this study includes virtually all singleton live births and
stillbirths in Sweden, Norway, Denmark (live births only), Finland, and Spain and births in
Canada are restricted to four provinces (Prince Edward Island, Ontario, Saskatchewan, and
British Columbia). Thus, the study offers the potential for findings to be generalizable to
countries with a similar distribution of risk factors. We employed a rigorous modeling strategy to
provide a comprehensive overview of age, period, and cohort effects for each country,
providing more nuance than traditional analyses of prevalence trends over time.
Maternal age is strongly associated with abruption, with young and especially older women at
substantially increased risk. Both social and biologic factors are likely to have played a role in
shaping abruption risk with respect to its association with maternal age. The increased
abruption risk among young women is likely influenced by socioeconomic factors. Young women
are less educated and of lower socioeconomic strata contributing to poor health. In turn, these
factors also adversely affect pregnancy, thereby increasing their abruption risk. On the other
hand, the strong association between increased maternal age and abruption is deeply rooted in
the biologic underpinnings. Placental underperfusion and uteroplacental ischemia strong
contributors to abruption [47,48] are commonly found in women of advanced age [49,50].
These women also carry less chance of successful reproduction, possibly due to aging uterine
effects due to repeated pregnancies. In sum, the strong effect of increased age on abruption
risk, independent of cohort and period effects, may reflect a pure biologic phenomenon is the
consequence of strong selection of high-risk mothers with advanced age or both.
With the exception of Canada and the US, abruption rates have been declining in all the
other countries in the 2000 (or earlier). In the US, the abruption rate initially increased up to
2000 and plateaued thereafter. One possible factor to explore in future studies in the
fortification of foods with folate, which occurred in the US and Canada in 1998 and may have led to
the decline (or plateauing) of abruption rates . Epidemiologic studies have reported
that women that took folate supplementation before or during the first trimester had a reduced
risk of abruption . Such universal folate fortification policy has not been implemented in
European countries. However the increased general awareness of the benefits of multivitamins
and folate in the prevention of neural tube defects and other pregnancy-related complications
may have led women to consume folate prepregnancy or during pregnancy.
Smoking during pregnancy is a strong risk factor for abruption (RRs 1.7 to 3.0), and may
have impacted abruption trends. Through a multilevel model, we tested if the recent decline in
abruption rates were the result of the declining smoking prevalence rates. Indeed, at least in the
US and Sweden, changing smoking rates were associated with between-period variation in
abruption rates, offering at least a partial explanation for the period effects observed in these
countries. While smoking does not fully explain the temporal decline in abruption rates, the
temporal decline in smoking rates may have helped in muting the increase in abruption rates.
What other factors might have contributed to the observed temporal decline in abruption
rates? Among the many risk factors that are associated with abruption [19,50,55], changes in
diagnostic criteria for abruption may have also played a role. There is no standardized
definition for abruption . It is therefore unlikely that differences in diagnostic criteria for
abruption may have led to differences in the prevalence rates of abruption across countries. We did
not detect a birth cohort effect associated with abruption. This suggests that the population
factors contributing to changes in abruption rates are those that have affected women of
childbearing age, rather than factors that have varied across age groups.
The strong maternal age effect on placental abruption is consistent across all seven countries in
this international comparison; these data do not show any evidence of a birth cohort effect.
Since 2000 (or earlier), abruption rates in Canada, Sweden, Norway, Denmark, Finland and
Spain show a sharp decline but the rates in the US since the 2000 have plateaued. These
observations suggest that factors that contribute to abruption may differ across countries. The
independent effects of maternal age and, to a lesser extent, period of birth, coupled with differences
in these determinants across countries point to the complicated dynamics that shape abruption
risk over time.
S1 Fig. Parameter estimates for smoking as a predictor of period effects for placental
abruption in three countries: Sweden, United States, and Canada.
Conceived and designed the experiments: CVA. Analyzed the data: AH KMK CVA. Wrote the
paper: CVA KMK AH MG CW SL MALF RSS MAW MT SC.
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