Jonathan Snowden

Implementation of G-Computation on a Simulated Data Set: Demonstration of a Causal Inference Technique

The growing body of work in the epidemiology literature focused on G-computation includes theoretical explanations of the method but very few simulations or examples of application. The small number of G-computation analyses in the epidemiology literature relative to other causal inference approaches may be partially due to a lack of didactic explanations of the method targeted toward an epidemiology audience. The authors provide a step-by-step demonstration of G-computation that is intended to familiarize the reader with this procedure. The authors simulate a data set and then demonstrate both G-computation and traditional regression to draw connections and illustrate contrasts between their implementation and interpretation relative to the truth of the simulation protocol. A marginal structural model is used for effect estimation in the G-computation example. The authors conclude by answering a series of questions to emphasize the key characteristics of causal inference techniques and the G-computation procedure in particular.

Elective induction of labor at term compared with expectant management: maternal and neonatal outcomes.

OBJECTIVE: To test the association of elective induction of labor at term compared with expectant management and maternal and neonatal outcomes. METHODS: This was a retrospective cohort study of all deliveries without prior cesarean delivery in California in 2006 using linked hospital discharge and vital statistics data. We compared elective induction at each term gestational age (37–40 weeks) as defined by The Joint Commission with expectant management in vertex, nonanomalous, singleton deliveries. We used multivariable logistic regression to test the association of elective induction and cesarean delivery, operative vaginal delivery, maternal third- or fourth-degree lacerations, perinatal death, neonatal intensive care unit admission, respiratory distress, shoulder dystocia, hyperbilirubinemia, and macrosomia (birth weight greater than 4,000 g) at each gestational week, stratified by parity. RESULTS: The cesarean delivery rate was 16%, perinatal mortality was 0.2%, and neonatal intensive care unit admission was 6.2% (N=362,154). The odds of cesarean delivery were lower among women with elective induction compared with expectant management across all gestational ages and parity (37 weeks [odds ratio (OR) 0.44, 95% confidence interval (CI) 0.34–0.57], 38 weeks [OR 0.43, 95% CI 0.38–0.50], 39 weeks [OR 0.46, 95% CI 0.41–0.52], 40 weeks [OR 0.57, CI 0.50–0.65]). Elective induction was not associated with increased odds of severe lacerations, operative vaginal delivery, perinatal death, neonatal intensive care unit admission, respiratory distress, shoulder dystocia, or macrosomia at any term gestational age. Elective induction was associated with increased odds of hyperbilirubinemia at 37 and 38 weeks of gestation and shoulder dystocia at 39 weeks of gestation. CONCLUSION: Elective induction of labor is associated with decreased odds of cesarean delivery when compared with expectant management. LEVEL OF EVIDENCE: II

The risk of intrauterine fetal death in the small-for-gestational-age fetus

OBJECTIVE We sought to evaluate the risk of intrauterine fetal death (IUFD) in small-for-gestational-age (SGA) fetuses. STUDY DESIGN We analyzed a retrospective cohort of all births in the United States in 2005, as recorded in a national database. We calculated the risk of IUFD within 3 sets of SGA threshold categories as well as within non-SGA pregnancies using the number of at-risk fetuses as the denominator. RESULTS The risk of IUFD increased with gestational age and was inversely proportional to percentile of birthweight for gestational age. The risk for IUFD in those <3rd percentile was as high as 58.0 IUFDs per 10,000 at-risk fetuses, 43.9 for <5th percentile, and 26.3 for <10th percentile compared to 5.1 for non-SGA gestations. CONCLUSION There is an increase in the risk of IUFD in SGA fetuses compared to non-SGA fetuses at all gestational ages with the greatest risk demonstrated in the lowest percentile cohort evaluated.

Planned Out-of-Hospital Birth and Birth Outcomes

BACKGROUND The frequency of planned out-of-hospital birth in the United States has increased in recent years. The value of studies assessing the perinatal risks of planned out-of-hospital birth versus hospital birth has been limited by cases in which transfer to a hospital is required and a birth that was initially planned as an out-of-hospital birth is misclassified as a hospital birth. METHODS We performed a population-based, retrospective cohort study of all births that occurred in Oregon during 2012 and 2013 using data from newly revised Oregon birth certificates that allowed for the disaggregation of hospital births into the categories of planned in-hospital births and planned out-of-hospital births that took place in the hospital after a womans intrapartum transfer to the hospital. We assessed perinatal morbidity and mortality, maternal morbidity, and obstetrical procedures according to the planned birth setting (out of hospital vs. hospital). RESULTS Planned out-of-hospital birth was associated with a higher rate of perinatal death than was planned in-hospital birth (3.9 vs. 1.8 deaths per 1000 deliveries, P=0.003; odds ratio after adjustment for maternal characteristics and medical conditions, 2.43; 95% confidence interval [CI], 1.37 to 4.30; adjusted risk difference, 1.52 deaths per 1000 births; 95% CI, 0.51 to 2.54). The odds for neonatal seizure were higher and the odds for admission to a neonatal intensive care unit lower with planned out-of-hospital births than with planned in-hospital birth. Planned out-of-hospital birth was also strongly associated with unassisted vaginal delivery (93.8%, vs. 71.9% with planned in-hospital births; P<0.001) and with decreased odds for obstetrical procedures. CONCLUSIONS Perinatal mortality was higher with planned out-of-hospital birth than with planned in-hospital birth, but the absolute risk of death was low in both settings. (Funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development.).

Induction of labor compared to expectant management in low-risk women and associated perinatal outcomes

OBJECTIVE We sought to examine the association of labor induction and perinatal outcomes. STUDY DESIGN This was a retrospective cohort study of low-risk nulliparous women with term, live births. Women who had induction at a given gestational age (eg, 39 weeks) were compared to delivery at a later gestation (eg, 40, 41, or 42 weeks). RESULTS Compared to delivery at a later gestational age, those induced at 39 weeks had a lower risk of cesarean (adjusted odds ratio [aOR], 0.90; 95% confidence interval [CI], 0.88-0.91) and labor dystocia (aOR, 0.88; 95% CI, 0.84-0.94). Their neonates had lowered risk of having 5-minute Apgar <7 (aOR, 0.81; 95% CI, 0.72-0.92), meconium aspiration syndrome (aOR, 0.30; 95% CI, 0.19-0.48), and admission to neonatal intensive care unit (aOR, 0.87; 95% CI, 0.78-0.97). Similar findings were seen for women who were induced at 40 weeks compared to delivery later. CONCLUSION Induction of labor in low-risk women at term is not associated with increased risk of cesarean delivery compared to delivery later.

Prevalence, correlates and trends in seroadaptive behaviours among men who have sex with men from serial cross-sectional surveillance in San Francisco, 2004-2011

Objectives We sought to assess the prevalence and correlates of seroadaptive behaviours (ie, sexual history incorporating some unprotected anal intercourse (UAI)) and conventional risk reduction behaviours (ie, consistent condom use or no anal intercourse) among men who have sex with men (MSM) in San Francisco in 2011. We compared the prevalence of seroadaptive behaviours between serial cross-sectional surveys from 2004, 2008 and 2011. Methods We analysed data from the 2011 wave of the National HIV Behavioral Surveillance system in San Francisco. We categorised mens self-reported sexual behaviour history in the past 6 months into a schema of seroadaptive behaviours and conventional risk reduction behaviours. We compared the prevalence of behaviour categories by self-reported HIV serostatus, HIV testing history, awareness of pre-exposure HIV prophylaxis (PrEP) and diagnosis of a sexually transmitted infection (STI). Results Seroadaptive behaviours remained common in San Francisco MSM, with a 2011 prevalence of 46.6%, up from 35.9% in 2004. Consistent condom use or no anal intercourse was more common than seroadaptive behaviours in HIV-negative MSM, men who had not heard of PrEP and men without an STI diagnosis. Seroadaptive behaviours increased from 2004 to 2011. Conclusions HIV seroadaptive behaviours remain common in San Francisco MSM, have increased in the last decade and are practiced differently by MSM with different sexual health knowledge and outcomes. Public health researchers and officials should continue to document the prevalence, intentionality, efficacy and safety of seroadaptive behaviours among diverse communities of MSM.

Selected perinatal outcomes associated with planned home births in the United States

OBJECTIVE More women are planning home birth in the United States, although safety remains unclear. We examined outcomes that were associated with planned home compared with hospital births. STUDY DESIGN We conducted a retrospective cohort study of term singleton live births in 2008 in the United States. Deliveries were categorized by location: hospitals or intended home births. Neonatal outcomes were compared with the use of the χ(2) test and multivariable logistic regression. RESULTS There were 2,081,753 births that met the study criteria. Of these, 12,039 births (0.58%) were planned home births. More planned home births had 5-minute Apgar score <4 (0.37%) compared with hospital births (0.24%; adjusted odds ratio, 1.87; 95% confidence interval, 1.36-2.58) and neonatal seizure (0.06% vs 0.02%, respectively; adjusted odds ratio, 3.08; 95% confidence interval, 1.44-6.58). Women with planned home birth had fewer interventions, including operative vaginal delivery and labor induction/augmentation. CONCLUSION Planned home births were associated with increased neonatal complications but fewer obstetric interventions. The trade-off between maternal preferences and neonatal outcomes should be weighed thoughtfully.

The impact of chronic hypertension and pregestational diabetes on pregnancy outcomes

OBJECTIVE The objective of the study was to examine the impact of chronic hypertension and pregestational diabetes on pregnancy outcomes. STUDY DESIGN This was a retrospective cohort study of 532,088 women undergoing singleton births in California in 2006. Women were categorized into chronic hypertension, pregestational diabetes, both, or neither. Pregnancy outcomes were compared using the χ(2) test and multivariable logistic regression to control for potential confounders. RESULTS We identified differences in perinatal outcomes between the groups. The rate of preterm birth in women with both conditions was 35.5% versus 25.5% in women with chronic hypertension versus 19.4% in women with pregestational diabetes (P < .001). The rate of small for gestational age was 18.2% in women with both versus 18.3% in women with chronic hypertension versus 9.7% in women with pregestational diabetes (P < .001). CONCLUSION The impact of having both chronic hypertension and pregestational diabetes in pregnancy varies, depending on the outcome examined. Although some had an additive effect (eg, stillbirth), others did not (eg, preeclampsia).

The role of ambient ozone in epidemiologic studies of heat-related mortality.

Background: A large and growing literature investigating the role of extreme heat on mortality has conceptualized the role of ambient ozone in various ways, sometimes treating it as a confounder, sometimes as an effect modifier, and sometimes as a co-exposure. Thus, there is a lack of consensus about the roles that temperature and ozone together play in causing mortality. Objectives: We applied directed acyclic graphs (DAGs) to the topic of heat-related mortality to graphically represent the subject matter behind the research questions and to provide insight on the analytical options available. Discussion: On the basis of the subject matter encoded in the graphs, we assert that the role of ozone in studies of temperature and mortality is a causal intermediate that is affected by temperature and that can also affect mortality, rather than a confounder. Conclusions: We discuss possible questions of interest implied by this causal structure and propose areas of future work to further clarify the role of air pollutants in epidemiologic studies of extreme temperature.

The association between hospital obstetric volume and perinatal outcomes in California

OBJECTIVE We sought to analyze the association between hospital obstetric volume and perinatal outcomes in California. STUDY DESIGN This was a retrospective cohort study of births occurring in California in 2006. Hospitals were divided into 4 obstetric volume categories. Unadjusted rates of neonatal mortality and birth asphyxia were calculated for each category, overall and among term deliveries with birthweight >2500 g. Multivariable logistic regression was used to control for confounders. Deliveries in rural hospitals were analyzed separately using different volume categories. RESULTS Prevalence of asphyxia increased with decreasing hospital volume overall and among term, non-low-birthweight infants, from 9/10,000 live births at highest-volume hospitals to 18/10,000 live births at the lowest-volume hospitals (P < .001). Similar trends were observed in rural hospitals, with rates increasing from 7-34/10,000 live births in low-volume rural hospitals (P < .001). CONCLUSION These findings provide evidence for an inverse association between hospital obstetric volume and birth asphyxia.