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Keywords:

  • Cohort study;
  • meconium aspiration syndrome;
  • obstetric interventions;
  • record linkage;
  • trends

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Analysis
  6. Results
  7. Discussion
  8. Conclusions
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. Acknowledgements
  14. References

Please cite this paper as: Vivian-Taylor J, Sheng J, Hadfield R, Morris J, Bowen J, Roberts C. Trends in obstetric practices and meconium aspiration syndrome: a population-based study. BJOG 2011;118:1601–1607.

Objective  To determine trends in the incidence of meconium aspiration syndrome (MAS), and maternal factors and obstetric practices associated with any decline.

Design  Population-based cohort study.

Setting  New South Wales (NSW), Australia.

Population  All 877 037 liveborn, singleton, term infants (≥37 weeks of gestation) in the period 1997–2007.

Methods  Data were obtained from birth records linked to the neonatal hospital discharge records. The birth data provided information on maternal and obstetric factors, whereas the outcome of interest, MAS, was obtained from hospital data on the neonates. Multivariable logistic regression was used to estimate the risk of MAS while simultaneously adjusting for the explanatory variables.

Main outcome measures  The incidence of MAS per 1000 births, and odds ratios and 95% confidence intervals for maternal and obstetric factors for the development of MAS.

Results  The incidence of MAS declined significantly by 11.3% per annum (95% CI 10.1–12.6; P < 0.001) from 4.1 per 1000 births in 1997 to 1.3 per 1000 births in 2007. This was associated with a statistically significant decline in risk factors: maternal smoking (from 20 to 12%), gestational age (from 57 to 47% ≥ 40 weeks of gestation), delivery at small hospitals (from 15 to 9%) and infants with birthweight below the third percentile (from 3.3 to 2.4%). There were simultaneous statistically significant increases in practices that reduce the risk of MAS: labour inductions (from 22 to 27%) and birth by caesarean section, both elective, prior to 40 weeks of gestation (from 7.3 to 13.8%), and emergency (from 3.0 to 5.3% prior to 40 weeks of gestation, and from 5.1 to 6.7% at 40 weeks of gestation or later).

Conclusions  The rate of MAS is declining, and this decline is associated with a reduction in maternal and pregnancy risk factors, and an increase in protective obstetric practices.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Analysis
  6. Results
  7. Discussion
  8. Conclusions
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. Acknowledgements
  14. References

Meconium aspiration syndrome (MAS) is a condition that largely affects term infants and leads to respiratory distress early in the neonatal period.1 It results from antenatal passage of meconium by the fetus, and subsequent aspiration in utero or with the neonate’s first breaths after birth.1,2 Meconium in the neonatal lungs initiates a complex pathophysiological response, including airway obstruction, airway inflammation, surfactant dysfunction and tissue necrosis, which in some cases is compounded by fetal acidosis and fetal depression at birth.2,3 The clinical consequences include hypoxaemia, acidosis, persistent pulmonary hypertension, a need for supplemental oxygen, and radio-opacities on chest X-ray.3

With approximately 4.6% of admissions to a neonatal intensive care unit or a special care nursery (NICU/SCN) at term resulting from MAS,4 MAS is an important cause of morbidity and mortality in the term neonate. Approximately one-third of neonates affected require intubation,5,6 and the mortality rate is 1.2–8.3%.5–7 Infants with MAS are also at increased risk of long-term neurological complications such as cerebral palsy and seizures.8,9

Antenatal or intrapartum factors associated with the development of MAS include advancing gestational age after 40 weeks of gestation, small for gestational age (SGA), intrapartum caesarean section and abnormal fetal heart rate patterns in labour.7,10,11 The associated neonatal characteristics, low Apgar scores at 1 and 5 minutes, low cord pH level, meconium below the vocal cords, and the need for intubation and ventilation,12–14 are likely to be a direct consequence of meconium aspiration or of a common pathological pathway. The rates of MAS started declining in the 1970s.5,7,11 It has been hypothesised that the changes in maternity care that have occurred during this time, including earlier gestation at delivery, increased delivery by caesarean section and increased intrapartum monitoring, are responsible.7,11 However, as the rate of MAS in the population is low (<0.8%),5,7,15 smaller clinical/chart review-based studies may not detect other concurrent changes in obstetric practices that influence the decline. Population health data are valuable for investigating this low incidence disorder and the factors that influence it. Therefore, the aim of this population-based study was to determine trends in the incidence of MAS, and maternal factors and obstetric practices associated with any decline.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Analysis
  6. Results
  7. Discussion
  8. Conclusions
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. Acknowledgements
  14. References

The study population comprised all liveborn, term (≥37 complete weeks of gestation), singleton infants born in New South Wales (NSW) hospitals during the period 1 January 1997–31 December 2007. NSW is the most populous state in Australia (∼7.1 million), and has over one-third of all Australian births (∼90 000 births per annum). The study period (1997–2007) was chosen because of the availability of linked birth and infant hospital records, including transfer admissions.

The population health data for this study were obtained from two validated, NSW Department of Health computerised data sets. The Midwives Data Collection (referred to as ‘birth data’) is a statutory population-based surveillance system that includes information on all births at ≥20 weeks of gestation or weighing at least 400 g. Information on maternal characteristics, pregnancy, labour and delivery, and infant outcomes are reported by the attending midwife or doctor. The Admitted Patients Data Collection (referred to as ‘hospital data’) is a census of all NSW inpatient hospital discharges (public and private). Diagnoses for each hospitalisation are coded from the medical records according to the International Classification of Diseases (ICD) version 9 for 1997–1998, and version 10 for 1999–2007. The NSW Department of Health provided anonymised, linked birth and hospital data for births during the study period. Over 98% of birth records linked to an infant hospital record. Only de-identified data were available to the researchers, and the study was approved on this basis by the NSW Population and Health Services Research Ethics Committee; case reviews of individual patient medical records are not permitted under this approval.

The outcome of interest was a diagnosis of MAS. Each linked birth–hospital record was searched in up to 20 ICD diagnosis fields for an MAS diagnosis code (770.1 in ICD9 and P24.0 in ICD10). The Australian Modification of the ICD requires ‘supplemental oxygen for a period of at least 24 hours’ for classification as MAS. In NSW MAS is reliably (96% agreement; κ = 0.8) and accurately (positive predictive value 95%) reported in the hospital data.16

Potential maternal, pregnancy and obstetric risk factors that are well and accurately reported in birth and/or hospital data were included in the analysis.17,18 The explanatory variables included year of birth, maternal age, parity, maternal smoking, any labour induction (medical and/or surgical), any use of oxytocin (for induction or augmentation of labour), fetal presentation, mode of delivery, infant sex, gestational age, size at birth and size of the hospital.7,10,11,13,14,19 Mode of delivery was categorised as normal vaginal, instrumental deliveries, caesarean section prior to the onset of labour (referred to as ‘elective’ caesarean section) and caesareans after the onset of labour (referred to as ‘emergency’ caesarean section). Hospital size was defined as tertiary, or non-tertiary large (>1000 births per year), medium (500–1000 births per year) and small (<500 births per year). Size at birth was classified into five groups using national birthweight percentiles.20

Other factors associated with MAS but probably symptoms of the disease included an Apgar score at 1 minute < 4, Apgar score at 5 minutes of < 7 and delivery room resuscitation (intermittent positive pressure respiration, intubation, ventilation and/or external cardiac massage). Missing data were infrequent on individual factors ranging from 0.03% for birthweight to 0.3% for smoking during pregnancy. Overall, only 0.9% of records were missing information on one or more of the factors in the analysis.

Analysis

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Analysis
  6. Results
  7. Discussion
  8. Conclusions
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. Acknowledgements
  14. References

First, we calculated and plotted the MAS incidence per 1000 births from 1997 to 2007. We used Poisson regression to estimate the average yearly change (with associated 95% confidence intervals, 95% CIs) in the number of infants with MAS, relative to 1997. Second, we examined changes in the frequency of maternal and pregnancy characteristics from 1997 to 2007 to determine population characteristics that changed significantly over time (χ2 for trend P < 0.01). Finally, contingency tables were used to analyse the crude relationship between MAS and time (year of birth), and maternal and delivery factors. Crude odds ratios (ORs) with 95% confidence intervals were estimated for the explanatory variables. Multivariable logistic regression was used to estimate the risk of MAS while simultaneously adjusting for the explanatory variables. Adjusted odds ratios were calculated by entering the proposed explanatory variables into the logistic regression model and retaining variables that were significant at P < 0.05 (Table 1). As the risk of MAS associated with elective caesarean section varied significantly by gestational age, the results for mode of delivery are stratified. The model fit was assessed using the Hosmer–Lemeshow goodness-of-fit test (P = 0.24). Analyses were carried out using sas 9.1 (SAS Institute, Cary, NC, USA).

Table 1.   Crude and adjusted odds ratios for risk factors for meconium aspiration syndrome (MAS)
CharacteristicNo. of births n = 877 037MAS (rate*) = 2149Crude OR (95% CI)Adjusted OR** (95% CI)
  1. *Rate of MAS per 1000 live term singleton births.

  2. **Adjusted for year of birth.

Maternal age (years)
<25169 380498 (2.9)1.16 (1.04–1.31)Not retained
25–29260 483659 (2.5)1.00 (referent)
30–35282 772610 (2.2)0.85 (0.76–0.95)
≥35164 056381 (2.3)0.92 (0.81–1.04)
Parity
0360 9991241 (3.4)1.96 (1.80–2.13)1.42 (1.29–1.56)
≥1516 038908 (1.8)1.00 (referent)1.00 (referent)
Smoking during pregnancy
No735 6651660 (2.3)1.00 (referent)1.00 (referent)
Yes139 200484 (3.5)1.54 (1.39–1.71)1.47 (1.32–1.63)
Hospital size
Tertiary226 991619 (2.7)1.28 (1.15–1.42)1.17 (1.05–1.30)
Large375 082799 (2.1)1.00 (referent)1.00 (referent)
Medium156 672381 (2.4)1.14 (1.01–1.29)1.03 (0.91–1.17)
Small101 158288 (2.8)1.34 (1.17–1.53)1.17 (1.02–1.35)
Labour induction
No induction658 2361696 (2.6)1.00 (referent)1.00 (referent)
Labour induction218 617451 (2.1)0.80 (0.72–0.89)0.61 (0.55–0.68)
Use of Oxytocin in labour
No Oxytocin588 6351323 (2.2)1.00 (referent)Not retained
Oxytocin207 536492 (2.4)1.06 (0.95–1.17)
Presentation
Cephalic842 8592091 (2.5)1.00 (referent)Not retained
Non–cephalic33 06153 (1.6)0.65 (0.49–0.85)
Delivery
‘Normal’ vaginal579 8321089 (1.9)1.00 (referent)See below
Elective caesarean112 042143 (1.3)0.68 (0.57–0.81)
Emergency caesarean90 459538 (5.9)3.18 (2.87–3.53)
Instrumental94 250379 (4.0)2.15 (1.91–2.41)
Gestational age (weeks)
3747 10266 (1.4)0.93 (0.71–1.21)See belowSee below
38143 222203 (1.4)0.94 (0.79–1.12)
39218 713330 (1.5)1.00 (referent)
40296 644787 (2.7)1.76 (1.55–2.00)
41151 474589 (3.9)2.58 (2.26–2.96)
≥4219 882174 (8.8)5.84 (4.86–7.03)
Mode of delivery and gestational age
<40 weeks
 Non–vaginal birth245 668323 (1.3)1.00 (referent)1.00 (referent)
 Elective caesarean92 32669 (0.7)0.57 (0.44–0.74)0.61 (0.47–0.79)
 Instrumental34 93590 (2.6)1.97 (1.56–2.48)1.89 (1.49–2.41)
 Emergency caesarean35 908117 (3.3)2.49 (2.01–3.07)2.49 (2.00–3.10)
≥40 weeks
 Non-vaginal birth334 164776 (2.3)1.75 (1.54–1.99)1.74 (1.52–1.99)
 Elective caesarean19 71674 (3.8)2.87 (2.23–3.69)2.63 (2.03–3.42)
 Instrumental59 315289 (4.9)3.73 (3.18–4.37)3.52 (2.98–4.17)
 Emergency caesarean54 551421 (7.7)5.92 (5.12–6.84)6.17 (5.29–7.20)
Baby’s sex
Male449 8751209 (2.7)1.22 (1.12–1.33)1.21 (1.11–1.32)
Female427 162940 (2.2)1.00 (referent)1.00 (referent)
Birthweight percentile
<3rd25 773173 (6.7)3.16 (2.70–3.70)2.78 (2.36–3.27)
3rd–9th60 704207 (3.4)1.60 (1.38–1.85)1.52 (1.31–1.76)
10th–90th701 3891496 (2.1)1.00 (referent)1.00 (referent)
>90th88 911266 (3.0)1.40 (1.23–1.60)1.41 (1.23–1.61)

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Analysis
  6. Results
  7. Discussion
  8. Conclusions
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. Acknowledgements
  14. References

From 1997 to 2007, among the 877 037 live singleton term births, there were 2149 infants with a diagnosis of MAS, giving an overall incidence rate of 2.5 cases per 1000 live singleton term births. The incidence of MAS declined significantly by 11.3% per annum (95% CI 10.1–12.6; P < 0.001), from 4.1 per 1000 births in 1997 to 1.3 in 2007 (Figure 1).

image

Figure 1.  Trend in the rate of meconium aspiration syndrome in New South Wales during the period 1997–2007.

Download figure to PowerPoint

Live singleton term births increased from 80 866 in 1997 to 85 593 in 2007. During this period there were also important changes in the maternity population and in obstetric practice. Maternal age increased such that the proportion of women aged ≥ 30 years rose (from 45 to 56%). There were statistically significant declines in maternal smoking (from 20 to 12%), delivery at ≥ 40 weeks of gestation (from 57 to 47%), including at ≥ 42 weeks of gestation (from 2.6 to 1.0%), and births in small hospitals (from 15 to 9%). Labour induction increased (from 22 to 27%), as did births by caesarean section. The proportion of births by elective caesarean section prior to 40 weeks of gestation increased (from 7.3 to 13.8%), as did births by emergency caesarean section (from 3.0 to 5.3% prior to 40 weeks of gestation, and from 5.1 to 6.7% at ≥ 40 weeks of gestation). There was a decline in the proportion of infants with birthweight for gestational age below the third percentile (from 3.3 to 2.4%) and below the tenth percentile (from 10.5 to 8.7%). The proportion of nullipara (average 41.4%) and instrumental deliveries (average 10.7%) did not change over the study period.

Crude and adjusted odds ratios for antenatal and delivery factors that may be associated with MAS are show in Table 1. Maternal factors independently associated with an increased risk of MAS were nulliparity, maternal smoking and birth in a tertiary or small hospital. Maternal smoking in pregnancy increased the risk of MAS in infants by 50% (adjusted OR 1.47, 95% CI 1.32–1.64; P < 0.001). The largest group of births took place in large non-tertiary hospitals (42.8%), and this group had lower rates of MAS (2.1 per 1000 births) compared with both tertiary hospitals (2.7 per 1000 births) and small hospitals (2.8 per 1000 births).

The obstetric practices associated with a reduced risk of MAS were elective caesarean section at <40 weeks of gestation and induction of labour (Table 1). After adjusting for other factors, induction of labour was associated with a reduced risk of MAS compared with women who were not induced, and elective caesarean section at <40 weeks of gestation was associated with a reduced risk of MAS compared with those born by normal vaginal birth. The obstetric procedures independently associated with an increased risk of MAS were delivery by emergency caesarean section and instrumental delivery. Overall, 42.7% of neonates with MAS were delivered by emergency caesarean section or instrumental delivery. The risk of developing MAS in neonates delivered by emergency caesarean section was more than double the risk for those delivered by normal vaginal birth. However, the rate of MAS among infants delivered by emergency caesarean declined by 13% per annum (95% CI 11–15; P < 0.001), from 10.9 per 1000 births in 1999 to 2.9 per 1000 births in 2007.

The fetal/neonatal characteristics independently associated with the development of MAS were male sex, gestational age ≥ 40 weeks and extremes of size at birth. The rate of MAS increased from 1.4 to 1.5 per 1000 births at 37–39 weeks of gestation to 2.7 per 1000 births at 40 weeks of gestation, 3.9 per 1000 births at 41 weeks of gestation and 8.8 per 1000 births at ≥ 42 weeks of gestation (Table 1). Compared with 10–90 birthweight percentiles infants, the risk of developing MAS was higher in small-for-gestational-age infants below the tenth centile, and was over two and a half times higher in neonates whose growth was less than the third centile for gestational age. As expected, other neonatal indicators strongly associated with MAS were an Apgar score of < 4 at 1 minute (OR 19.2, 95% CI 17.4–21.2) and <7 at 5 minute (OR 27, 95% CI 24.4–29.8), and the need for neonatal resuscitation (OR 49.1; 95% CI 43.6–55.4).

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Analysis
  6. Results
  7. Discussion
  8. Conclusions
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. Acknowledgements
  14. References

Meconium aspiration syndrome (MAS) has decreased in the NSW birth population from 4.11 per 1000 term births in 1997 to 1.30 per 1000 term births in 2007. Important changes in maternal and fetal population factors and obstetric practices between 1997 and 2007 provide some explanation for this decrease. Smoking during pregnancy, a significant maternal risk factor for MAS, declined over the study period. Protective obstetrics practices such as earlier gestational age at delivery, induction of labour, elective caesarean section prior to 40 weeks of gestation and delivery in large non-tertiary hospitals all increased. Fetal growth restriction is a well-recognised risk factor for MAS in the term infant,7,10,14 which is supported by our study. Therefore, it is significant that the number of infants below the third centile, and between the third and ninth centiles, at birth decreased over the study period. This may be attributable to maternal factors, such as the decline in smoking, or possibly to better identification of babies that are growth restricted, such that they are born earlier, by induction of labour or elective caesarean section, before crossing to the lower centiles.

The decline in MAS is a trend that commenced in the 1970s.5,7,11 Given the significant morbidity and mortality caused by MAS in the term neonate, this trend contributes to improvements in neonatal outcomes. An earlier study cited changes in neonatal practices as a possible explanation for the decline in MAS.5 There was a significant change in the study period, away from both the immediate suctioning of the baby’s oropharynx on delivery of the baby’s head, and subsequent visualisation of the vocal cords at birth for all meconium-exposed neonates. Direct visualisation of the vocal cords was abandoned for babies with good cardiorespiratory effort at birth. These practice changes were as a direct result of evidence from two large trials that demonstrated such practices were not associated with a reduction in MAS or an overall benefit to the neonate.21,22 It is noteworthy that MAS has declined throughout the period since such practices were discontinued.

The passage of meconium prior to birth occurs more commonly with advancing gestational age.23 We saw a decline in the number of births after 42 weeks of gestation, when this risk is highest. Routine amniotic fluid index (AFI) screening of low-risk women after 40 weeks of gestation to reduce adverse perinatal outcomes, such as MAS, has become common practice. Despite low sensitivity and the resultant increase in interventions, such as induction of labour, this practice may be contributing to the decline in MAS in the population.24 Whereas exposure to meconium-stained liquor was not available in our data, the trend to earlier gestational age at delivery is likely to result in fewer neonates at risk. Earlier delivery is one factor driving the reduction in MAS, and thus improvements in neonatal outcomes.

Our findings demonstrate a protective effect of elective caesarean section prior to 40 weeks of gestation and of labour induction on the development of MAS. Labour induction reduced the risk of MAS by 40%. Such a protective effect of labour induction was surprising and unexpected. As it was independent of gestational age it suggests that the benefit results from mechanisms other than early delivery. We propose that the practices associated with the induction of labour (fetal monitoring and timely delivery for ‘abnormal’ fetal heart-rate patterns) are the explanation. Approximately one-third of all emergency caesarean sections are performed because of a concern over fetal well-being, usually in the context of an abnormal fetal heart-rate pattern recorded on a cardiotocograph (CTG).25,26 The complex interaction between in utero acidosis, meconium-stained liquor and the subsequent development of MAS, explains the strong association between delivery by emergency caesarean section and MAS. The number of women delivering by emergency caesarean section increased between 1997 and 2007. In support of the idea that intrapartum practices are resulting in the earlier delivery of the ‘at risk’ fetus, is the decline in the rate of MAS in neonates delivered by emergency caesarean section. Therefore, despite being strongly associated with MAS, in practice, the increase in the emergency caesarean section rate is likely to be contributing to the overall decline.

The size of the delivery hospital has not been looked at previously with respect to MAS rates. The changing nature of obstetric practice in Australia has seen a move away from delivery units with fewer than 500 births per year.27 Excluding tertiary hospitals who have a ‘high risk’ case mix, our study found increasing rates of MAS as the hospital size decreased. Smaller hospitals are less likely to have obstetric and paediatric staff on site, or 24-hour access to immediate operative delivery. Fewer deliveries in these hospitals may explain the findings and be contributing to the declining rates of MAS in this study. Although this would suggest that combining hospitals that perform less than 500 births per year could improve neonatal outcomes, in rural areas this could impact on safety through increasing patient travelling distances during labour.

The strength of this study is the use of recent, validated, linked population health data, which allows the study of uncommon outcomes such as MAS and its risk factors. By studying the whole population we can estimate trends over time that are generalisable. However, we could not identify pregnancies with evidence of fetal distress, and nor was there information about the presence of meconium-stained liquor or CTG monitoring and interpretation. The complex nature of intrapartum fetal physiology, and the management decisions that result, are not captured. We found neonatal morbidity outcomes (low Apgar scores and neonatal resuscitation) were highly associated with the development of MAS in this cohort of term neonates. This is consistent with the findings of previous studies.7,14 However, they have been excluded from the final adjusted model because they are usually manifestations of the disorder rather than precipitating events.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Analysis
  6. Results
  7. Discussion
  8. Conclusions
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. Acknowledgements
  14. References

This population-based study provides important information about the potential modifiable factors that could be targeted to further improve neonatal outcomes at an individual and population level. Further reductions in maternal smoking, improved detection of SGA at term, and structuring maternity services around 24-hour obstetric and paediatric care could be targeted. However, MAS has now become an uncommon neonatal outcome and cannot be considered in isolation. The overall maternal and neonatal risks associated with reducing it further may outweigh the benefits, if elective caesarean section and deliveries prior to 39 weeks of gestation increase.

Contribution to authorship

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Analysis
  6. Results
  7. Discussion
  8. Conclusions
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. Acknowledgements
  14. References

JVT, CLR, JMM and JRB developed the concept and design of the study. JVT and CLR were responsible for the overall drafting of the article. JSC and RMH conducted the analysis and drafted the methods and results. JMM and JRB revised the manuscript for important intellectual content. All authors contributed to the interpretation of data and had final approval of the manuscript to be published.

Details of ethics approval

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Analysis
  6. Results
  7. Discussion
  8. Conclusions
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. Acknowledgements
  14. References

Only de-identified data were available to the researchers, and the study was approved on this basis by the NSW Population and Health Services Research Ethics Committee, on 14 July 2006 (ref. no. 2006-06-011).

Funding

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Analysis
  6. Results
  7. Discussion
  8. Conclusions
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. Acknowledgements
  14. References

This work was supported by an Australian National Health and Medical Research Council (NHMRC) Project Grant (#457302). Christine Roberts is supported by an NHMRC Senior Research Fellowship (#457078).

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Analysis
  6. Results
  7. Discussion
  8. Conclusions
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. Acknowledgements
  14. References

We thank the NSW Department of Health and the NSW Centre for Health Record Linkage for record linkage, and for access to the population health data.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Analysis
  6. Results
  7. Discussion
  8. Conclusions
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. Acknowledgements
  14. References
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