Contribution of Changing Risk Factors to the Trend in Breech Presentation
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1 The final version of this paper was published in Australian New Zealand Journal of Obstetrics and Gynaecology 2016 56(6): 564-570 MANUSCRIPT TITLE: Contribution of Changing Risk Factors to the Trend in Breech Presentation at Term SHORT TITLE: Trend in Term Breech Presentation WORD COUNTS Abstract= 247/250 Main text= 2262/2500 References=30/30 Tables=2/4 Figures=2 KEYWORDS (MESH TERMS) (1) breech presentation, (2) trends, (3) risk factors, (4) external cephalic version 2 ABSTRACT Background: Recent population-wide changes in perinatal risk factors may affect rates of breech presentation at birth, and have implications for the provision of breech services and clinical training in breech management. Aims: To determine the trend in breech presentation at term and investigate whether changes in maternal and pregnancy characteristics explain the observed trend. Materials and Methods: All singleton term (≥37 week) births in New South Wales during 2002 – 2012 were identified through birth and associated hospital records. Annual rates of breech presentation were determined. Logistic regression modelling was used to predict expected rates of breech presentation over time and these were compared with observed rates. A priori predictors included maternal age, country of birth, parity, smoking during pregnancy, diabetes, pregnancy hypertension, placenta praevia, previous singleton term breech, previous caesarean section, infant sex, gestational age, birthweight, and congenital anomalies. Hospital and Medicare data were used to assess trends in external cephalic version. Results: Among 914,147 singleton term births, 3.1% were breech at delivery. Rates declined from 3.6% in 2002 to 2.7% in 2012 (test for trend p<0.001). Breech presentation was predicted to increase from 3.6% in 2002 to 4.3% in 2012 because of increased maternal age, nulliparity, maternal diabetes, history of breech presentation and previous caesarean section. Use of external cephalic version appears to have increased over time. Conclusions: Breech presentation at delivery has decreased in New South Wales. Increased use of external cephalic version likely accounts for this decline, as changes in risk factors do not. 3 INTRODUCTION Breech presentation occurs in 2 to 4% of births at term [1, 2] and is associated with greater perinatal mortality and morbidity than vertex presentation [3, 4]. Risk factors for breech presentation include older maternal age [5-7], nulliparity [5-7], pre- existing diabetes [7, 8], gestational diabetes [7], smoking [5], placenta previa [7], uterine abnormalities [6, 9], previous breech presentation [10], previous caesarean section [5], small fetal size [5-7], and congenital anomalies [6, 7, 10]. There is also some evidence that the rate of breech presentation may differ by ethnicity, with women of African ancestry appearing to have the lowest rates [5, 8]. In New South Wales (NSW), breech presentation was reported to be stable at 3.4% from the 1990s to the early 2000s [11, 12]. However, there have been substantial changes in maternal and pregnancy characteristics over recent years [13]: with increases in maternal age, nulliparity, previous caesarean section, diabetes, placenta previa, and early term birth likely to result in increased rates of breech presentation. On the other hand, population trends that might lead to lower rates of breech presentation include increases in fetal size at birth, mothers born outside Australia, and a decline in maternal smoking [14]. Furthermore, anecdotal reports suggest increased use of external cephalic version (ECV) which has the potential to reduce the rate of breech presentation at birth by 30% or more [15]. Changes in the rate of breech risk factors and the provision of ECV will affect the rate of breech presentation, with implications for service provision, quality of care, and clinical training. The aim of the current study was to examine the trend in breech presentation at term in NSW and to determine how known risk factors have contributed to this trend. A secondary aim was to report on concurrent trends in the use of ECV. 4 MATERIALS AND METHODS Study population The study population included all singleton births at term (37 – 42 completed weeks of gestation) during the 11-year period from 1st January 2002 to 31st December 2012 in NSW, Australia. Data sources Data for this study were sourced from two routinely collected administrative datasets. Breech presentation and risk factors for breech presentation were identified from the NSW Perinatal Data Collection (birth records) and the NSW Admitted Patient Data Collection (hospital records). The birth records describe all births in NSW of at least 20 weeks gestation or at least 400g birth weight. The birth records are completed by an attending midwife or medical practitioner and include information on maternal health, pregnancy, labour, delivery, and infant characteristics. The hospital records are a census of discharges, transfers and deaths from NSW public and private hospitals. Diagnoses and procedures associated with each hospital record are coded by trained medical coders according to the International Statistical Classification of Diseases and Related Health Problems, Tenth Revision, Australian Modification (ICD-10-AM) [16] and the Australian Classification of Health Interventions [17], respectively. The birth records and maternal and infant hospital records were linked by the NSW Centre for Health Record Linkage (http://www.cherel.org.au/) using probabilistic record linkage. It has been shown that probabilistic linkage has a high rate of accuracy 5 [18]. To preserve privacy, personal identifiers were removed before the data were provided to the authors. A linkage key was provided for the authors to merge the relevant birth and hospital records for the current study. Ethics approval for data linkage and the study was obtained from the NSW Population Health Services Research Ethics Committee. There is no single data source capturing the provision of ECV. Information on inpatients undergoing ECV is recorded in the hospital records and we extracted these to examine the number of ECV procedures conducted over the study period. We also used Medicare Item Reports to determine the number of ECV procedures (item code “16501”) billed to Medicare for women aged 15 – 54 years in NSW during the study period [19]. The overlap between hospital and Medicare data is unknown. Outcome and Predictors Breech presentation at birth was the study outcome of interest and was recorded in the birth records. This was compared with all other presentations at birth. Predictors were selected a priori based on literature review and specialist knowledge of risk factors for breech presentation. Information on risk factors was identified from the birth and/or hospital records [20, 21]. Risk factors considered were: maternal age (in years), parity (nulliparous/multiparous), maternal country of birth was used as a proxy for maternal ethnicity and categorised using the Standard Australian Classification of Countries major groups (Oceania/North West Europe/Southern and Eastern Europe/North Africa and Middle East/South East Asia/North Ease Asia/Southern and Central Asia/Americas/Subsaharan Africa) [22], smoking during pregnancy (none/any), previous caesarean section (no/yes), previous term singleton breech (no/yes), maternal 6 diabetes including pre-existing and gestational diabetes (none/any), pregnancy hypertension (no/yes), placenta praevia (no/yes), infant sex (male/female), gestational age (37/38/39/40/41/42 weeks), birthweight for gestational age and sex (<10th- small for gestational age, 10-90th, >90th percentile- large for gestational age) [23], and infant congenital anomalies (none/any). Congenital anomalies were identified from the infant’s hospital record at birth. Statistical analyses The rate of breech presentation per year was calculated for 2002 – 2012. Changes in the overall rate of the predictors were tabulated and tests for trends using the Wald chi-square were conducted. Relative change was calculated using [(2012 rate- 2002 rate) / (2002 rate)] * 100. To examine the impact of changing risk factors on the trend in breech presentation, the study population was split into 2 datasets: development data from the year 2002 and prediction data from the years 2003 to 2012. Logistic regression was used to model the association between breech presentation and the risk factors using 2002 data. All a priori identified risk factors were included in modelling irrespective of crude associations. This predictive model was applied to subsequent years to predict the expected rate of breech presentation based on actual changes in the risk factors. The predicted rates were compared to the observed rates of breech presentation. ECVs per 100 term deliveries in the study population were calculated based on the number of ECV records in the hospital and Medicare data for NSW. All analyses were conducted using SAS 9.3 (SAS Institute, NC). 7 RESULTS Breech presentations accounted for 3.1% of 914,147 term births during the 11- year study period. The observed rate of breech presentation decreased from 3.6% to 2.7% between 2002 and 2012 (test for trend p<0.001) (Figure 1). The risk factors for breech presentation changed over time (Table 1). Compared to a decade earlier, women who gave birth in 2012 were significantly older, more likely to have been born overseas, nulliparous, have diabetes or placenta praevia. Multiparous women were more likely to have a history of caesarean section and previous