Perinatal Death Classification Research Articles

Causes of stillbirth in ethnically diverse women in a Perth metropolitan hospital: A retrospective study.

Most published reports analysing the differences in causation of stillbirth between different ethnic groups focus on stillbirth risk factors, with a paucity of data comparing actual causes of stillbirth. To determine whether causes of stillbirth differ between Caucasian and non-Caucasian ethnic groups in an Australian context. Data from all stillbirths occurring at 20 or more completed weeks of gestation between 1 January 2010 and 31 December 2020 at a secondary level, outer metropolitan hospital, were analysed in this retrospective case series. Causes of stillbirth as determined by perinatal autopsy and placental histopathology were categorised using the Perinatal Society of Australia and New Zealand Perinatal Death Classification and compared between Caucasian and non-Caucasian groups. Ninety-two stillbirths (0.7% of all births) were identified during the study period. A greater proportion of non-Caucasian women had small for gestation age placentas compared to Caucasian women (n = 22/43 (51%) vs n = 12/49 (24%); P = 0.025). A greater proportion of stillbirths were caused by hypoxic peripartum death in non-Caucasian than in Caucasian women (n = 4/43 (9%) vs n = 0/49 (0%); P = 0.044), and a greater prevalence of placental dysfunction was seen in the non-Caucasian cohort compared to Caucasian women (n = 14/43 (33%) vs n = 8/49 (16%); P = 0.057). The differences observed in causes of stillbirth between Caucasian and non-Caucasian women are hypothesis generating and warrant further larger-scale, multi-centred studies using standardised definitions and classification systems to determine whether these differences persist in a more representative sample.

Analysis of perinatal mortality at a teaching hospital in Dar es Salaam, Tanzania, 1999-2003.

We conducted a retrospective analysis of perinatal mortality at Muhimbili National Hospital (MNH), Dar es Salaam, Tanzania 1999-2003 in order to categorise/classify perinatal deaths as well as to identify key factors in perinatal care that could be improved. Data were retrieved from the MNH obstetric database and causes of early neonatal deaths were traced from the neonatal ward register. The study includes all foetuses weighing =500g. A modified Nordic-Baltic classification was used for classification of perinatal deaths. Over a 5-year period there were 77,815 babies born with a perinatal mortality rate of 124 per 1000 births, 78% of which was labour related stillbirth. The PMR was 913/1000 for singleton births and 723/1000 for multiple births for babies weighing less than 1500 grams and 65/1000 for singleton births and 116/1000 for multiple births for babies weighing 2500 grams or more. Babies weighing less than 1500 grams contributed 26% of PMR, whereas 41% occurred in babies weighing 2500 grams or more. The majority (79%) of neonatal deaths had Apgar score <7 at 5 minutes and the most common causes of neonatal mortality were birth asphyxia (37%) and prematurity (29%). Labour related deaths were more common in multiple pregnancies. The majority of the perinatal deaths should be essentially avoidable through improved quality of intrapartum care. Establishment of perinatal audit at MNH can help identify key actions for improved care.

Perinatal death audit and classification of stillbirths in two provinces in Papua New Guinea: A retrospective analysis

To undertake a retrospective perinatal death audit and assessment of avoidable factors associated with stillbirths among a cohort of women in two provinces in Papua New Guinea. We used data from an ongoing cluster-randomized crossover trial in 10 sites among 4600 women in Papua New Guinea (from 2017 to date). The overarching aim is to improve birth outcomes. All stillbirths from July 2017 to January 2020 were identified. The Perinatal Problem Identification Program was used to analyze each stillbirth and review associated avoidable factors. There were 59 stillbirths among 2558 births (23 per 1000 births); 68% (40/59) were classified "fresh" and 32% as "macerated". Perinatal cause of death was identified for 63% (37/59): 30% (11/37) were due to intrapartum asphyxia and traumatic breech birth and 19% (7/37) were the result of pre-eclampsia. At least one avoidable factor was identified for 95% (56/59) of stillbirths. Patient-associated factors included lack of response to reduced fetal movements and delay in seeking care during labor. Health personnel-associated factors included poor intrapartum care, late diagnosis of breech presentation, and prolonged second stage with no intervention. Factors associated with stillbirths in this setting could be avoided through a package of interventions at both the community and health-facility levels.

Results of the perinatal audit of the Northwestern Federal District

Aim. To assess the results of a perinatal audit of the Northwestern Federal District and to identify opportunities for a decrease in perinatal mortality.&#x0D; Methods. The audit of perinatal loss was conducted in two stages: (1) remote audit audit of perinatal mortality indicators; (2) medical history audit audit of cases of perinatal death of a child based on medical documentation. Held the copy of the data from the 925 medical records for 220 cases of perinatal death. The perinatal audit of the Northwestern Federal District used the Nordic-Baltic perinatal death classification. The following statistical methods were used for statistical data processing: incidence rate of a trait was determined by using frequency tables, the statistical significance of differences was tested by using contingency tables, the Chi-square criterion, along with the Pearson correlation coefficient. The statistical significance of differences in quantitative indicators was assessed by using Student's t-Test. The significance level was set at p 0.05.&#x0D; Results. It was found that in the Northwestern Federal District pregnancy losses III category of the Nordic-Baltic classification (gestational age newborn, more than 28 weeks, without congenital malformations and intrauterine growth restriction) is 27.5%, intranatal losses VI category of the Nordic-Baltic classification (gestational age newborn, more than 28 weeks, without congenital malformations and intrauterine growth restriction) 7.4%, the loss of newborns VIIIXI category of the Nordic-Baltic classification (gestational age newborn, more than 28 weeks, without congenital malformations and intrauterine growth restriction) 16.9%. Among children who died during the perinatal period, children of gestational age over 28 weeks significantly predominate (p=0.003). In the nosological structure of stillbirth, most of the diseases are associated with respiratory disorders (85.9%), infectious complications are 14.1%. The main causes of death of newborns in the early neonatal period are respiratory disorders 40.0% and infectious diseases specific to the perinatal period 36.0%. The assessment of the sexual prevalence of pregnancy losses did not reveal a statistically significant difference (p=0.29). The assessment of the sexual characteristics of intranatal losses showed that boys significantly predominate (p=0.003).&#x0D; Conclusion. The perinatal audit revealed that, in the Northwestern Federal District, the level of the mobile reserve of perinatal losses associated with managed causes is 51.8%.

Prediction of perinatal death using machine learning models: a birth registry-based cohort study in northern Tanzania

ObjectiveWe aimed to determine the key predictors of perinatal deaths using machine learning models compared with the logistic regression model.DesignA secondary data analysis using the Kilimanjaro Christian Medical Centre (KCMC)...

Improving Diagnostic Classification of Stillbirths and Neonatal Deaths Using ICD-PM (International Classification of Diseases for Perinatal Mortality) Codes: Validation Study.

BackgroundStillbirths and neonatal deaths have long been imperfectly classified and recorded worldwide. In Hong Kong, the current code system is deficient (>90% cases with unknown causes) in providing the diagnoses of perinatal mortality cases.ObjectiveThe objective of this study was to apply the International Classification of Diseases for Perinatal Mortality (ICD-PM) system to existing perinatal death data. Further, the aim was to assess whether there was any change in the classifications of perinatal deaths compared with the existing classification system and identify any areas in which future interventions can be made.MethodsWe applied the ICD-PM (with International Statistical Classification of Diseases and Related Health Problems, 10th Revision) code system to existing perinatal death data in Kwong Wah Hospital, Hong Kong, to improve diagnostic classification. The study included stillbirths (after 24 weeks gestation) and neonatal deaths (from birth to 28 days). The retrospective data (5 years) from May 1, 2012, to April 30, 2017, were recoded by the principal investigator (HML) applying the ICD-PM, then validated by an overseas expert (EA) after she reviewed the detailed case summaries. The prospective application of ICD-PM from May 1, 2017, to April 30, 2019, was performed during the monthly multidisciplinary perinatal meetings and then also validated by EA for agreement.ResultsWe analyzed the data of 34,920 deliveries, and 119 cases were included for analysis (92 stillbirths and 27 neonatal deaths). The overall agreement with EA of our codes using the ICD-PM was 93.2% (111/119); 92% (78/85) for the 5 years of retrospective codes and 97% (33/34) for the 2 years of prospective codes (P=.44). After the application of the ICD-PM, the overall proportion of unknown causes of perinatal mortality dropped from 34.5% (41/119) to 10.1% (12/119) of cases (P<.001).ConclusionsUsing the ICD-PM would lead to a better classification of perinatal deaths, reduce the proportion of unknown diagnoses, and clearly link the maternal conditions with these perinatal deaths.

Why correctly identifying maternal condition in perinatal death classification systems is crucial: a commentary.

Why correctly identifying maternal condition in perinatal death classification systems is crucial: a commentary.

A qualitative perinatal audit at a rural health facility in Eldoret, Kenya

Background: Objectives of the study were to ascertain the pattern of occurrence of perinatal mortality by applying the World Health Organization (WHO), International Classification of Diseases, tenth revision (ICD-10) to deaths during the perinatal period, ICD perinatal mortality (ICD-PM), following the introduction of a qualitative perinatal audit process at a rural health facility in Kenya.Methods: A single centre retrospective analysis demonstrating the application of the WHO, ICD-PM. Data pertaining to perinatal deaths for the period from 1st May 2017 to 31st August 2018 was obtained from Plateau Mission Hospital perinatal audit records.Results: There were 22 perinatal deaths during the study period, 17 were included in the study. The overall perinatal death rate was 11 per 1000 births. Antepartum deaths were as a consequence of fetal growth related problems (33.3%), infection (33.3%) or unexplained (33.3%) with pregnancy-related hypertensive disorders (gestational hypertension, pre-eclampsia and eclampsia) being the most frequent medical condition associated with the mortalities. Neonatal deaths (47.1%) were the most frequent in the study and were a consequence of low birth weight and prematurity (25.0%), Convulsions and disorders of cerebral status (25.0%). The maternal condition in most of these cases being complications of placenta, cord and membranes. Acute intrapartum events and were least in this setting accounting for 17.4% of deaths.Conclusions: The ICD-PM is generalizable and its use in perinatal death classification emphasises focus on both mother and baby. Our study showed the majority of perinatal deaths occurred in the early neonatal period &amp; affected mostly preterm infants.

Determining the cause of stillbirth in Kumasi, Ghana.

To classify cause-of-death (COD) for stillbirths occurring in a major referral hospital in Kumasi, Ghana. In a retrospective review conducted between June 8, 2011, and June 12, 2012, detailed information was collected on all stillbirths delivered at Komfo Anokye Teaching Hospital in Kumasi, Ghana. Patient records were independently reviewed by investigators using the Perinatal Society of Australia and New Zealand's Perinatal Death Classification system to determine COD for each case. COD was analyzed in 465 stillbirth cases. The leading causes of death were hypoxic interpartum death (105, 22.6%), antepartum hemorrhage (67, 14.4%), hypertension (52, 11.2%), and perinatal infection (32, 6.9%). One hundred and fifty seven (33.8%) stillbirths were classified as unexplained antepartum deaths. This evaluation of stillbirth in a busy, tertiary care hospital in Kumasi, Ghana provides crucial insight into the high volume of stillbirth in Ghana as well as its medical causes. The study demonstrated the high rate of stillbirth attributed to hypoxic intrapartum events, placental abruption, pre-eclampsia, and unspecified bacterial infections. Yet, our rate of unexplained stillbirths underscores the need for a stillbirth classification system that thoughtfully integrates the needs and limitations of low-resource settings as unexplained stillbirth rates are a common indicator of the effectiveness of a classification system.

A prospective review of perinatal mortality at Hospital Nacional Guido Valadares (HNGV).

Timor-Leste has one of the highest perinatal mortality rates in the Asia-Pacific region. Consistent and accurate data collection improves understanding of perinatal outcomes and facilitates the development of interventions to reduce stillbirths and early neonatal deaths. (1) To identify changes in the rates of stillbirth and early neonatal deaths from previous published data. (2) To determine if prospective data collection and the application of the simplified Causes Of Death and Associated Conditions (CODAC) classification allows better identification of perinatal deaths in Timor-Leste. A prospective audit of perinatal deaths of women delivering at Hospital Nacional Guido Valadares (HNGV) was undertaken from January to June 2016 inclusive. The hospital birth registry, maternal and neonatal records were reviewed to determine the most likely aetiology and classification of perinatal deaths using the simplified CODAC system. One hundred and ten stillbirths and 28 early neonatal deaths were identified. Fifty-four percent of perinatal deaths occurred antepartum, 26% intrapartum and 20% were early neonatal deaths. Cause of death among stillbirths could not be ascertained in 40% of cases. Intrapartum asphyxia was the commonest identified aetiology of intrapartum and early neonatal deaths. There has been limited improvement in the rate of stillbirths and early neonatal deaths at HNGV. Intrapartum hypoxia and maternal hypertensive conditions were the most common identified aetiologies highlighting areas where targeted interventions may help reduce high perinatal mortality rates. Aetiology of perinatal deaths, particularly antepartum stillbirths was difficult to discern even when well-tested classification systems are used.

Tracking facility-based perinatal deaths in Tanzania: Results from an indicator validation assessment.

BackgroundGlobally, an estimated 2.7 million babies die in the neonatal period annually, and of these, about 0.7 million die from intrapartum-related events. In Tanzania 51,000 newborn deaths and 43,000 stillbirths occur every year. Approximately two-thirds of these deaths could be potentially prevented with improvements in intrapartum and neonatal care. Routine measurement of fetal intrapartum deaths and newborn deaths that occur in health facilities can help to evaluate efforts to improve the quality of intrapartum care to save lives. However, few examples exist of indicators on perinatal mortality in the facility setting that are readily available through health management information systems (HMIS).MethodsFrom November 2016 to April 2017, health providers at 10 government health facilities in Kagera region, Tanzania, underwent refresher training on perinatal death classification and training on the use of handheld Doppler devices to assess fetal heart rate upon admission to maternity services. Doppler devices were provided to maternity services at the study facilities. We assessed the validity of an indicator to measure facility-based pre-discharge perinatal mortality by comparing perinatal outcomes extracted from the HMIS maternity registers to a gold standard perinatal death audit.ResultsSensitivity and specificity of the HMIS neonatal outcomes to predict gold standard audit outcomes were both over 98% based on analysis of 128 HMIS–gold standard audit pairs. After this validation, we calculated facility perinatal mortality indicator from HMIS data using fresh stillbirths and pre-discharge newborn death as the numerator and women admitted in labor with positive fetal heart tones as the denominator. Further emphasizing the validity of the indicator, FPM values aligned with expected mortality by facility level, with lowest rates in health centers (range 0.3%– 0.5%), compared to district hospitals (1.5%– 2.9%) and the regional hospital (4.2%).ConclusionThis facility perinatal mortality indicator provides an important health outcome measure that facilities can use to monitor levels of perinatal deaths occurring in the facility and evaluate impact of quality of care improvement activities.

Classification of perinatal deaths according to ICD-PM: an audit on perinatal post-mortems in a tertiary care centre in Sri Lanka

Introduction: Ending preventable perinatal deaths have become a priority in global public health agenda, highlighting the importance of collecting quality data related to perinatal mortality. Pathological post-mortems and placental examination play a vital role in identifying the cause of death. The objective of this analysis was to classify causes of death according to a uniform classification system, enabling comparison of data in different settings. Congenital malformations were further categorized according to the involved organ system. Method: This is a retrospective analysis of post-mortems of stillbirths (including the placentas) and neonatal deaths already reported at the Department of Pathology, University of Colombo from 2009-2015. Autopsy and placental examination were performed according to standard protocols. The cause of death was classified according to ICD-PM. Results: The study included 291 post - mortems; 198 stillbirths, 7 intra-partum deaths and 86 neonatal deaths. Ante-partum hypoxia was the commonest cause of death in stillbirths (83/198=41.9%), while, congenital malformations was the commonest in intra-partum (5/7=71.4%) and neonatal deaths (38/86=44.2%). Low birth weight and prematurity is the second most common cause in neonatal deaths (24/86=27.9%). Malformations were commonly present in cardiovascular system (stillbirths: 22/47=47%, intra-partum and neonatal deaths: 31/62=50%) followed by respiratory system (stillbirths: 14/47=30%, intra-partum and neonatal deaths: 20/62=32%). Conclusion: Ante-partum hypoxia, congenital malformations and low birth weight and prematurity are the leading causes of death in stillbirths and neonates however, quality data of a prospective study, analyzing maternal and feto/infant pathology together, is required to improve the validity of the results.

Audit of stillbirths in a Nigerian teaching hospital

Objective: The purpose of this study is to classify the stillbirths (SBs) in our institution and to determine the avoidable contributors using a pro forma that was developed in our perinatal unit. Materials and Method: All mothers who delivered SBs at Obafemi Awolowo University Hospitals Complex Ile-Ife, Nigeria, from April 2014 to August 2015 were the subjects of this study. At our weekly perinatal mortality meeting, every SB was reviewed in detail to determine the cause of death and identify gaps in the care. The causes of fetal death were classified using three classification systems – Simplified CODAC, ReCoDe, and our newly developed Ife Perinatal Death Classification System. Remote contributors to fetal demise were assigned using a purpose–designed “Avoidable Factor Proforma” A SB was deemed avoidable if one or more factors were identified. Results: The total number of deliveries during the study was 2,142 with 109 SBs giving a gross SB rate of 51/1000 total births. The SB rate was significantly lower among booked patients than their unbooked counterparts (15 vs. 140 per 1000; P Conclusion: Our SB rate remains unacceptably high, with a preponderance of antepartum deaths among booked patients. Majority of the SBs are avoidable dictating the need for training in modern methods of fetal surveillance and improved efficiency of our health-delivery systems.

The WHO application of ICD-10 to deaths during the perinatal period (ICD-PM): results from pilot database testing in South Africa and United Kingdom.

To apply the World Health Organization (WHO) Application of the International Classification of Diseases, tenth revision (ICD-10) to deaths during the perinatal period: ICD-Perinatal Mortality (ICD-PM) to existing perinatal death databases. Retrospective application of ICD-PM. South Africa, UK. Perinatal death databases. Deaths were grouped according to timing of death and then by the ICD-PM cause of death. The main maternal condition at the time of perinatal death was assigned to each case. Causes of perinatal mortality, associated maternal conditions. In South Africa 344/689 (50%) deaths occurred antepartum, 11% (n=74) intrapartum and 39% (n=271) in the early neonatal period. In the UK 4377/9067 (48.3%) deaths occurred antepartum, with 457 (5%) intrapartum and 4233 (46.7%) in the neonatal period. Antepartum deaths were due to unspecified causes (59%), chromosomal abnormalities (21%) or problems related to fetal growth (14%). Intrapartum deaths followed acute intrapartum events (69%); neonatal deaths followed consequences of low birthweight/ prematurity (31%), chromosomal abnormalities (26%), or unspecified causes in healthy mothers (25%). Mothers were often healthy; 53%, 38% and 45% in the antepartum, intrapartum and neonatal death groups, respectively. Where there was a maternal condition, it was most often maternal medical conditions, and complications of placenta, cord and membranes. The ICD-PM can be a globally applicable perinatal death classification system that emphasises the need for a focus on the mother-baby dyad as we move beyond 2015. ICD-PM is a global system that classifies perinatal deaths and links them to maternal conditions.

Characteristics of a global classification system for perinatal deaths: a Delphi consensus study.

BackgroundDespite the global burden of perinatal deaths, there is currently no single, globally-acceptable classification system for perinatal deaths. Instead, multiple, disparate systems are in use world-wide. This inconsistency hinders accurate estimates of causes of death and impedes effective prevention strategies. The World Health Organisation (WHO) is developing a globally-acceptable classification approach for perinatal deaths. To inform this work, we sought to establish a consensus on the important characteristics of such a system.MethodsA group of international experts in the classification of perinatal deaths were identified and invited to join an expert panel to develop a list of important characteristics of a quality global classification system for perinatal death. A Delphi consensus methodology was used to reach agreement. Three rounds of consultation were undertaken using a purpose built on-line survey. Round one sought suggested characteristics for subsequent scoring and selection in rounds two and three.ResultsThe panel of experts agreed on a total of 17 important characteristics for a globally-acceptable perinatal death classification system. Of these, 10 relate to the structural design of the system and 7 relate to the functional aspects and use of the system.ConclusionThis study serves as formative work towards the development of a globally-acceptable approach for the classification of the causes of perinatal deaths. The list of functional and structural characteristics identified should be taken into consideration when designing and developing such a system.

Rethinking placental pathology in the PSANZ classification of unexplained stillbirth at term.

Approximately 30 per cent of stillbirths are currently classified 'unexplained' using the Perinatal Society of Australia and New Zealand (PSANZ) classification system in New Zealand. This unexplained category includes deaths with placental pathology even though the importance of placental pathology and its causal relationship to stillbirth is well described. To determine whether unexplained stillbirths in New Zealand classified using PSANZ criteria can be more usefully classified based on placental pathology. Audit of the classification of cause of death among 'unexplained antepartum death' at term by perinatal pathologist review of postmortem and/or placental pathology reports using the current PSANZ Perinatal Death Classification (PDC)10 classification and a proposed 'significant placental pathology' subclassification. The main outcome measure was a change in cause of death from unexplained term stillbirth to an alternative PSANZ classification or to significant placental pathology subcategory. In total, 177 unexplained stillbirths with a postmortem and/or placental pathology report in New Zealand between 2007 and 2013 inclusive were reviewed. Twenty-three cases (13%) had significant placental pathology that could have been a direct cause of the stillbirth. A further seven cases (4%) were misclassified and could be better classified within another PDC category. A classification system incorporating placental pathologies which are recognised by the current literature to be causative of stillbirth would better describe stillbirths at term in New Zealand. This would benefit parental counselling and follow-up in subsequent pregnancies. A standard approach to reporting placental pathology would benefit clinicians. Education on placental pathology for clinicians working with parents experiencing stillbirth and multidisciplinary approach to classification is also recommended.

Ascertaining Cause of Perinatal Deaths in A Tertiary Care Hospital

Perinatal mortality rate of Nepal is high indicating poor obstetrics and neonatal care services in the country. Identifying the causes of perinatal deaths would help in identifying the avoidable deaths so that timely interventions can be implemented in future. This study was conducted with the aim of finding out the perinatal mortality rate and its causes at our centre, Manipal Teaching hospital. This is a prospective observational type of study. Perinatal deaths after 28 weeks of gestation and weighing more than 1 kg and neonatal deaths within one week of birth were included. Maternal and neonatal characteristics were studied. Perinatal deaths were classified according to Aberdeen classification of perinatal deaths. Results indicated perinatal mortality rate of 37.6 per 1000 births with more than 60% stillbirths. Mechanical factors during intrapartum period (21.5%) and neonatal factors like prematurity, meconium aspiration and sepsis (22.4%) were major causes of the perinatal mortality. Almost 24% of the perinatal deaths were unexplained and these occurred in macerated stillbirths. Therefore, focus should be on providing quality antenatal care and strict intrapartum surveillance as well as neonatal care to bring down the perinatal deaths. Autopsy should be considered when cause of death is unexplained.

Term perinatal mortality audit in the Netherlands 2010–2012: a population-based cohort study

ObjectiveTo assess the implementation and first results of a term perinatal internal audit by a standardised method.DesignPopulation-based cohort study.SettingAll 90 Dutch hospitals with obstetric/paediatric departments linked to community practices of...

PPO.11 Perinatal mortality in Ireland: A national clinical audit

BackgroundPerinatal mortality has decreased in high-resource countries but cause of death, especially for stillbirths, is often unexplained. The Irish National Perinatal Epidemiology Centre (NPEC) established a national clinical audit on...

Variation in classification of live birth with newborn period death versus fetal death at the local level may impact reported infant mortality rate.

BackgroundTo better understand factors that may impact infant mortality rates (IMR), we evaluated the consistency across birth hospitals in the classification of a birth event as either a fetal death or an early neonatal (infant) death using natality data from North Carolina for the years 1995–2000.MethodsA database consisting of fetal deaths and infant deaths occurring within the first 24 hours after birth was constructed. Bivariate, followed by multivariable regression, analyses were used to control for relevant maternal and infant factors. Based upon hospital variances, adjustments were made to evaluate the impact of the classification on statewide infant mortality rate.ResultsAfter controlling for multiple maternal and infant factors, birth hospital remained a factor related to the classification of early neonatal versus fetal death. Reporting of early neonatal deaths versus fetal deaths consistent with the lowest or highest hospital strata would have resulted in an adjusted IMR varying from 7.5 to 10.64 compared with the actual rate of 8.95.ConclusionsValid comparisons of IMR among geographic regions within and between countries require consistent classification of perinatal deaths. This study demonstrates that local variation in categorization of death events as fetal death versus neonatal death within the first 24 hours after delivery may impact a state-level IMR in a meaningful magnitude. The potential impact of this issue on IMRs should be examined in other state and national populations.