Real World Data are Not Always Big Data: The Case for Primary Data Collection on Medication Use in Pregnancy in the Context of Birth Defects Research.

Many examples of the use of real-world data in the area of pharmacoepidemiology include "big data" such as insurance claims, medical records, or hospital discharge databases. However, "big" is not always better, particularly when studying outcomes with narrow windows of etiologic relevance. Birth defects are one such outcome, where specificity of exposure timing is critical. Studies with primary data collection can be designed to query details on the timing of medication use, as well as type, dose, frequency, duration, and indication, that can better characterize the "real world". Because birth defects are rare, etiologic studies are typically case-control in design, like the National Birth Defects Prevention Study, Birth Defects Study to Evaluate Pregnancy exposureS, and Slone Birth Defects Study. Recall bias can be a concern, but the ability to collect detailed information on both prescription and over-the-counter medication use and on other exposures such as diet, family history, and sociodemographic factors is a distinct advantage over claims and medical record data sources. Case-control studies with primary data collection are essential to advancing the pharmacoepidemiology of birth defects.

Public health priorities for gastroschisis: Summary of a meeting sponsored by the Centers for Disease Control and Prevention and the March of Dimes.

BACKGROUND: Gastroschisis has increased worldwide over several decades; however, there are significant gaps in understanding risk factors for development of the defect, particularly those that might be modifiable. Despite advances in survival, little is known about longer-term outcomes for affected individuals. METHODS: On April 27- and 28, 2023, the National Center on Birth Defects and Developmental Disabilities at the Centers for Disease Control and Prevention (CDC) and March of Dimes sponsored a meeting entitled "Public Health Priorities for Gastroschisis". The meeting goals were to review current knowledge on gastroschisis, discuss research gaps, and identify future priorities for public health surveillance, research, and action related to gastroschisis. Meeting participants encompassed a broad range of expertise and experience, including public health, clinical care of individuals with gastroschisis, affected individuals and families, and representatives from professional organizations and federal agencies. RESULTS: Several goals were identified for future public health surveillance and research, including focused theory-driven research on risk factors and increased study of longer-term effects of gastroschisis through improved surveillance. Certain public health actions were identified, that which could improve the care of affected individuals, including increased education of providers and enhanced resources for patients and families. CONCLUSIONS: These efforts may lead to an improved understanding of pathogenesis, risk factors, and outcomes and to improved care throughout the lifespan.

Maternal exposure to zolpidem and risk of specific birth defects.

Zolpidem is a non-benzodiazepine agent indicated for treatment of insomnia. While zolpidem crosses the placenta, little is known about its safety in pregnancy. We assessed associations between self-reported zolpidem use 1 month before pregnancy through to the end of the third month ("early pregnancy") and specific birth defects using data from two multi-site case-control studies: National Birth Defects Prevention Study and Slone Epidemiology Center Birth Defects Study. Analysis included 39,711 birth defect cases and 23,035 controls without a birth defect. For defects with ≥ 5 exposed cases, we used logistic regression with Firth's penalised likelihood to estimate adjusted odds ratios and 95% confidence intervals, considering age at delivery, race/ethnicity, education, body mass index, parity, early-pregnancy antipsychotic, anxiolytic, antidepressant use, early-pregnancy opioid use, early-pregnancy smoking, and study as potential covariates. For defects with three-four exposed cases, we estimated crude odds ratios and 95% confidence intervals. Additionally, we explored differences in odds ratios using propensity score-adjustment and conducted a probabilistic bias analysis of exposure misclassification. Overall, 84 (0.2%) cases and 46 (0.2%) controls reported early-pregnancy zolpidem use. Seven defects had sufficient sample size to calculate adjusted odds ratios, which ranged from 0.76 for cleft lip to 2.18 for gastroschisis. Four defects had odds ratios > 1.8. All confidence intervals included the null. Zolpidem use was rare. We could not calculate adjusted odds ratios for most defects and estimates are imprecise. Results do not support a large increase in risk, but smaller increases in risk for certain defects cannot be ruled out.

Maternal physical activity, sitting, and risk of non-cardiac birth defects.

BACKGROUND: The relationship between maternal physical activity (PA)/sitting and birth defects is largely unexplored. We examined whether pre-pregnancy PA/sitting were associated with having a pregnancy affected by a birth defect. METHODS: We used data from two United States population-based case-control studies: 2008-2011 deliveries from the National Birth Defects Prevention Study (NBDPS; 9 states) and 2014-2018 deliveries from the Birth Defects Study To Evaluate Pregnancy exposureS (BD-STEPS; 7 states). Cases with one of 12 non-cardiac birth defects (n = 3798) were identified through population-based registries. Controls (n = 2682) were live-born infants without major birth defects randomly sampled using vital/hospital records. Mothers self-reported pre-pregnancy PA/sitting. Unconditional logistic regression models estimated associations between PA/sitting categories and the 12 birth defects. RESULTS: Mothers engaging in pre-pregnancy PA was associated with a reduced odds of five (spina bifida, cleft palate, anorectal atresia, hypospadias, transverse limb deficiency) and a higher odds of two (anencephaly, gastroschisis) birth defects. Mothers spending less time sitting in pre-pregnancy was associated with a reduced odds of two (anorectal atresia, hypospadias) and a higher odds of one (cleft lip with or without cleft palate) birth defect. CONCLUSIONS: Reasonable next steps include replication of these findings, improved exposure assessment, and elucidation of biologic mechanisms. IMPACT: Using data from two population-based case-control studies, we found that mothers engaging in different types of physical activity in the 3 months before pregnancy had an infant with a reduced odds of five and a higher odds of two birth defects. Mothers spending less time sitting in the 3 months before pregnancy had an infant with a reduced odds of two and a higher odds of one birth defect. Clarification and confirmation from additional studies are needed using more precise exposure measures, distinguishing occupational from leisure-time physical activity, and elucidation of mechanisms supporting these associations.

Rare variants in CAPN2 increase risk for isolated hypoplastic left heart syndrome.

Hypoplastic left heart syndrome (HLHS) is a severe congenital heart defect (CHD) characterized by hypoplasia of the left ventricle and aorta along with stenosis or atresia of the aortic and mitral valves. HLHS represents only ∼4%-8% of all CHDs but accounts for ∼25% of deaths. HLHS is an isolated defect (i.e., iHLHS) in 70% of families, the vast majority of which are simplex. Despite intense investigation, the genetic basis of iHLHS remains largely unknown. We performed exome sequencing on 331 families with iHLHS aggregated from four independent cohorts. A Mendelian-model-based analysis demonstrated that iHLHS was not due to single, large-effect alleles in genes previously reported to underlie iHLHS or CHD in >90% of families in this cohort. Gene-based association testing identified increased risk for iHLHS associated with variation in CAPN2 (p = 1.8 × 10-5), encoding a protein involved in functional adhesion. Functional validation studies in a vertebrate animal model (Xenopus laevis) confirmed CAPN2 is essential for cardiac ventricle morphogenesis and that in vivo loss of calpain function causes hypoplastic ventricle phenotypes and suggest that human CAPN2707C>T and CAPN21112C>T variants, each found in multiple individuals with iHLHS, are hypomorphic alleles. Collectively, our findings show that iHLHS is typically not a Mendelian condition, demonstrate that CAPN2 variants increase risk of iHLHS, and identify a novel pathway involved in HLHS pathogenesis.

Neighborhood Deprivation and Neural Tube Defects.

BACKGROUND: Individual measures of socioeconomic status (SES) have been associated with an increased risk of neural tube defects (NTDs); however, the association between neighborhood SES and NTD risk is unknown. Using data from the National Birth Defects Prevention Study (NBDPS) from 1997 to 2011, we investigated the association between measures of census tract SES and NTD risk. METHODS: The study population included 10,028 controls and 1829 NTD cases. We linked maternal addresses to census tract SES measures and used these measures to calculate the neighborhood deprivation index. We used generalized estimating equations to calculate adjusted odds ratios (aORs) and 95% confidence intervals (CIs) estimating the impact of quartiles of census tract deprivation on NTDs adjusting for maternal race-ethnicity, maternal education, and maternal age at delivery. RESULTS: Quartiles of higher neighborhood deprivation were associated with NTDs when compared with the least deprived quartile (Q2: aOR = 1.2; 95% CI = 1.0, 1.4; Q3: aOR = 1.3, 95% CI = 1.1, 1.5; Q4 (highest): aOR = 1.2; 95% CI = 1.0, 1.4). Results for spina bifida were similar; however, estimates for anencephaly and encephalocele were attenuated. Associations differed by maternal race-ethnicity. CONCLUSIONS: Our findings suggest that residing in a census tract with more socioeconomic deprivation is associated with an increased risk for NTDs, specifically spina bifida.

Is gastroschisis associated with county-level socio-environmental quality during pregnancy?

BACKGROUND: Gastroschisis prevalence more than doubled between 1995 and 2012. While there are individual-level risk factors (e.g., young maternal age, low body mass index), the impact of environmental exposures is not well understood. METHODS: We used the U.S. Environmental Protection Agency's Environmental Quality Index (EQI) as a county-level estimate of cumulative environmental exposures for five domains (air, water, land, sociodemographic, and built) and overall from 2006 to 2010. Adjusted odds ratios (aOR) and 95% confidence interval (CI) were estimated from logistic regression models between EQI tertiles (better environmental quality (reference); mid; poorer) and gastroschisis in the National Birth Defects Prevention Study from births delivered between 2006 and 2011. Our analysis included 594 cases with gastroschisis and 4105 infants without a birth defect (controls). RESULTS: Overall EQI was modestly associated with gastroschisis (aOR [95% CI]: 1.29 [0.98, 1.71]) for maternal residence in counties with poorer environmental quality, compared to the reference (better environmental quality). Within domain-specific indices, only the sociodemographic domain (aOR: 1.51 [0.99, 2.29]) was modestly associated with gastroschisis, when comparing poorer to better environmental quality. CONCLUSIONS: Future work could elucidate pathway(s) by which components of the sociodemographic domain or possibly related psychosocial factors like chronic stress potentially contribute to risk of gastroschisis.

Inpatient Hospitalization Costs Associated with Birth Defects Among Persons Aged <65 Years - United States, 2019.

Changing treatments and medical costs necessitate updates to hospitalization cost estimates for birth defects. The 2019 National Inpatient Sample was used to estimate the service delivery costs of hospitalizations among patients aged <65 years for whom one or more birth defects were documented as discharge diagnoses. In 2019, the estimated cost of these birth defect-associated hospitalizations in the United States was $22.2 billion. Birth defect-associated hospitalizations bore disproportionately high costs, constituting 4.1% of all hospitalizations among persons aged <65 years and 7.7% of related inpatient medical costs. Updating estimates of hospitalization costs provides information about health care resource use associated with birth defects and the financial impact of birth defects across the life span and illustrates the need to determine the continued health care needs of persons born with birth defects to ensure optimal health for all.

Patterns of Prescription Medication Use during the First Trimester of Pregnancy in the United States, 1997-2018.

The objective of this analysis was to describe patterns of prescription medication use during pregnancy, including secular trends, with consideration of indication, and distributions of use within demographic subgroups. We conducted a descriptive secondary analysis using data from 9,755 women whose infants served as controls in two large United States case-control studies from 1997-2011 and 2014-2018. After excluding vitamin, herbal, mineral, vaccine, i.v. fluid, and topical products and over-the-counter medications, the proportion of women that reported taking at least one prescription medication in the first trimester increased over the study years, from 37% to 50% of women. The corresponding proportions increased with increasing maternal age and years of education, were highest for non-Hispanic White women (47%) and lowest for Hispanic women (24%). The most common indication for first trimester use of a medication was infection (12-15%). Increases were observed across the years for medications used for indications related to nausea/vomiting, depression/anxiety, infertility, thyroid disease, diabetes, and epilepsy. The largest relative increase in use among women was observed for medications to treat nausea/vomiting, which increased from 3.8% in the earliest years of the study (1997-2001) to 14.8% in 2014-2018, driven in large part by ondansetron use. Prescription medication use in the first trimester of pregnancy is common and increasing. Many medical conditions require treatments among pregnant women, often involving pharmacotherapy, which necessitates consideration of the risk and safety profiles for both mother and fetus.

Identification of pregnancies and their outcomes in healthcare claims data, 2008-2019: An algorithm.

Pregnancy is a condition of broad interest across many medical and health services research domains, but one not easily identified in healthcare claims data. Our objective was to establish an algorithm to identify pregnant women and their pregnancies in claims data. We identified pregnancy-related diagnosis, procedure, and diagnosis-related group codes, accounting for the transition to International Statistical Classification of Diseases, 10th Revision, Clinical Modification (ICD-10-CM) diagnosis and procedure codes, in health encounter reporting on 10/1/2015. We selected women in Merative MarketScan commercial databases aged 15-49 years with pregnancy-related claims, and their infants, during 2008-2019. Pregnancies, pregnancy outcomes, and gestational ages were assigned using the constellation of service dates, code types, pregnancy outcomes, and linkage to infant records. We describe pregnancy outcomes and gestational ages, as well as maternal age, census region, and health plan type. In a sensitivity analysis, we compared our algorithm-assigned date of last menstrual period (LMP) to fertility procedure-based LMP (date of procedure + 14 days) among women with embryo transfer or insemination procedures. Among 5,812,699 identified pregnancies, most (77.9%) were livebirths, followed by spontaneous abortions (16.2%); 3,274,353 (72.2%) livebirths could be linked to infants. Most pregnancies were among women 25-34 years (59.1%), living in the South (39.1%) and Midwest (22.4%), with large employer-sponsored insurance (52.0%). Outcome distributions were similar across ICD-9 and ICD-10 eras, with some variation in gestational age distribution observed. Sensitivity analyses supported our algorithm's framework; algorithm- and fertility procedure-derived LMP estimates were within a week of each other (mean difference: -4 days [IQR: -13 to 6 days]; n = 107,870). We have developed an algorithm to identify pregnancies, their gestational age, and outcomes, across ICD-9 and ICD-10 eras using administrative data. This algorithm may be useful to reproductive health researchers investigating a broad range of pregnancy and infant outcomes.

Maternal dietary caffeine consumption and risk of birth defects in the National Birth Defects Prevention Study, 1997-2011.

BACKGROUND: Caffeine consumption is common during pregnancy, but published associations with birth defects are mixed. We updated estimates of associations between prepregnancy caffeine consumption and 48 specific birth defects from the National Birth Defects Prevention Study (NBDPS) for deliveries from 1997 to 2011. METHODS: NBDPS was a large population-based case-control study conducted in 10 U.S. states. We categorized self-reported total dietary caffeine consumption (mg/day) from coffee, tea, soda, and chocolate as: <10, 10 to <100, 100 to <200, 200 to <300, and ≥ 300. We used logistic regression to estimate adjusted odds ratios (aORs [95% confidence intervals]). Analyses for defects with ≥5 exposed case children were adjusted for maternal race/ethnicity, age at delivery, body mass index, early pregnancy cigarette smoking and alcohol use, and study site. RESULTS: Our analysis included 30,285 case and 11,502 control children, with mothers of 52% and 54%, respectively, reporting consuming <100 mg caffeine, and 11% of mothers of both cases and controls reported consuming ≥300 mg per day. Low (10 to <100 mg/day) levels of prepregnancy caffeine consumption were associated with statistically significant increases in aORs (1.2-1.7) for 10 defects. Associations with high (≥300 mg/day) levels of caffeine were generally weaker, except for craniosynostosis and aortic stenosis (aORs = 1.3 [1.1-1.6], 1.6 [1.1-2.3]). CONCLUSIONS: Given the large number of estimates generated, some of the statistically significant results may be due to chance and thus the weakly increased aORs should be interpreted cautiously. This study supports previous observations suggesting lack of evidence for meaningful associations between caffeine consumption and the studied birth defects.

Exome-wide assessment of isolated biliary atresia: A report from the National Birth Defects Prevention Study using child-parent trios and a case-control design to identify novel rare variants.

The etiology of biliary atresia (BA) is unknown, but recent studies suggest a role for rare protein-altering variants (PAVs). Exome sequencing data from the National Birth Defects Prevention Study on 54 child-parent trios, one child-mother duo, and 1513 parents of children with other birth defects were analyzed. Most (91%) cases were isolated BA. We performed (1) a trio-based analysis to identify rare de novo, homozygous, and compound heterozygous PAVs and (2) a case-control analysis using a sequence kernel-based association test to identify genes enriched with rare PAVs. While we replicated previous findings on PKD1L1, our results do not suggest that recurrent de novo PAVs play important roles in BA susceptibility. In fact, our finding in NOTCH2, a disease gene associated with Alagille syndrome, highlights the difficulty in BA diagnosis. Notably, IFRD2 has been implicated in other gastrointestinal conditions and warrants additional study. Overall, our findings strengthen the hypothesis that the etiology of BA is complex.

Reproductive Health of Women with Congenital Heart Defects.

This report provides an overview of the unique reproductive health issues facing women with congenital heart defects (CHDs) and of the clinical care and professional guidelines on contraception, preconception care, and pregnancy for this population. It describes Centers for Disease Control and Prevention (CDC) activities related to surveillance of reproductive health issues among females with CHDs. It also describes CDC's work bringing awareness to physicians who provide care to adolescents and women with CHDs, including obstetrician/gynecologists, about the need for lifelong cardiology care for their patients with CHDs.

Delayed entry into prenatal care among women with pre-pregnancy health conditions, National Birth Defects Prevention Study, 1997-2011.

First trimester entry into prenatal care is recommended for all women, and especially women with pre-pregnancy conditions. Our objective was to determine whether women with pre-pregnancy conditions were at lower risk of entry after the first trimester (delayed entry) into prenatal care than women without a pre-pregnancy health condition. We used data from 10,890 participants in the National Birth Defects Prevention Study who delivered liveborn infants without birth defects. Women reported pre-pregnancy conditions and timing of entry into prenatal care during a computer-assisted telephone interview. Multivariable logistic regression analyses were conducted to evaluate whether having a pre-pregnancy condition was associated with delayed entry into prenatal care compared to women without pre-pregnancy conditions. Approximately 13% of women reported delayed entry into prenatal care, and 18% of women reported a pre-pregnancy condition. Delayed entry into prenatal care was not associated with pre-pregnancy cardiometabolic or neurologic conditions. Women with thyroid conditions were less likely to report delayed entry into prenatal care (prevalence odds ratio (OR), 95% confidence interval (CI): 0.55 [0.32, 0.94]), but women with hematologic and respiratory conditions were more likely to report delayed entry into prenatal care (OR: 1.95 [1.00, 3.82] and 1.27 [0.95, 1.72], respectively), compared to those without any chronic conditions. Future research investigating the success of early prenatal care among women with thyroid conditions could identify ways to reduce delayed prenatal care among women with other pre-pregnancy conditions.

Interaction of maternal medication use with ambient heat exposure on congenital heart defects in the National Birth Defects Prevention Study.

BACKGROUND: Maternal exposure to weather-related extreme heat events (EHEs) has been associated with congenital heart defects (CHDs) in offspring. Certain medications may affect an individual's physiologic responses to EHEs. We evaluated whether thermoregulation-related medications modified associations between maternal EHE exposure and CHDs. METHODS: We linked geocoded residence data from the U.S. National Birth Defects Prevention Study, a population-based case-control study, to summertime EHE exposures. An EHE was defined using the 90th percentile of daily maximum temperature (EHE90) for each of six climate regions during postconceptional weeks 3-8. Adjusted odds ratios (aORs) and 95% confidence intervals (CIs) for associations between EHE90 and the risk of CHDs were estimated by strata of maternal thermoregulation-related medication use and climate region. Interaction effects were evaluated on multiplicative and additive scales. RESULTS: Over 45% of participants reported thermoregulation-related medication use during the critical period of cardiogenesis. Overall, these medications did not significantly modify the association between EHEs and CHDs. Still, medications that alter central thermoregulation increased aORs (95% CI) of EHE90 from 0.73 (0.41, 1.30) among non-users to 5.09 (1.20, 21.67) among users in the Southwest region, U.S. This effect modification was statistically significant on the multiplicative (P = 0.03) and additive scales, with an interaction contrast ratio (95% CI) of 1.64 (0.26, 3.02). CONCLUSION: No significant interaction was found for the maternal use of thermoregulation-related medications with EHEs on CHDs in general, while medications altering central thermoregulation significantly modified the association between EHEs and CHDs in Southwest U.S. This finding deserves further research.

Exome sequencing identifies genetic variants in anophthalmia and microphthalmia.

Anophthalmia and microphthalmia (A/M) are rare birth defects affecting up to 2 per 10,000 live births. These conditions are manifested by the absence of an eye or reduced eye volumes within the orbit leading to vision loss. Although clinical case series suggest a strong genetic component in A/M, few systematic investigations have been conducted on potential genetic contributions owing to low population prevalence. To overcome this challenge, we utilized DNA samples and data collected as part of the National Birth Defects Prevention Study (NBDPS). The NBDPS employed multi-center ascertainment of infants affected by A/M. We performed exome sequencing on 67 family trios and identified numerous genes affected by rare deleterious nonsense and missense variants in this cohort, including de novo variants. We identified 9 nonsense changes and 86 missense variants that are absent from the reference human population (Genome Aggregation Database), and we suggest that these are high priority candidate genes for A/M. We also performed literature curation, single cell transcriptome comparisons, and molecular pathway analysis on the candidate genes and performed protein structure modeling to determine the potential pathogenic variant consequences on PAX6 in this disease.

Is maternal employment site a source of exposure misclassification in studies of environmental exposures and birth outcomes? A simulation-based bias analysis of haloacetic acids in tap water and hypospadias.

In population research, exposure to environmental contaminants is often indirectly assessed by linking residence to geocoded databases of environmental exposures. We explored the potential for misclassification of residence-based environmental exposure as a result of not accounting for the workplace environments of employed pregnant women using data from a National Birth Defects Prevention Study (NBDPS) analysis of drinking water haloacetic acids and hypospadias. Methods: The original analysis used NBDPS data from women with haloacetic acid exposure information in eight states who delivered an infant with second- or third-degree hypospadias (cases) or a male infant without a birth defect (controls) between 2000 and 2005. In this bias analysis, we used a uniform distribution to randomly select 11%-14% of employed women that were assumed to change municipal water systems between home and work and imputed new contaminant exposures for tap water beverages consumed at work among the selected women using resampled values from the control population. Multivariable logistic regression was used to estimate the association between hypospadias and haloacetic acid ingestion with the same covariates and exposure cut-points as the original study. We repeated this process across 10,000 iterations and then completed a sensitivity analysis of an additional 10,000 iterations where we expanded the uniform distribution (i.e., 0%, 28%). Results: In both simulations, the average results of the 10,000 iterations were nearly identical to those of the initial study. Conclusions: Our results suggest that household estimates may be sufficient proxies for worksite exposures to haloacetic acids in tap water.

Exome sequencing identifies variants in infants with sacral agenesis.

BACKGROUND: Sacral agenesis (SA) consists of partial or complete absence of the caudal end of the spine and often presents with additional birth defects. Several studies have examined gene variants for syndromic forms of SA, but only one has examined exomes of children with non-syndromic SA. METHODS: Using buccal cell specimens from families of children with non-syndromic SA, exomes of 28 child-parent trios (eight with and 20 without a maternal diagnosis of pregestational diabetes) and two child-father duos (neither with diagnosis of maternal pregestational diabetes) were exome sequenced. RESULTS: Three children had heterozygous missense variants in ID1 (Inhibitor of DNA Binding 1), with CADD scores >20 (top 1% of deleterious variants in the genome); two children inherited the variant from their fathers and one from the child's mother. Rare missense variants were also detected in PDZD2 (PDZ Domain Containing 2; N = 1) and SPTBN5 (Spectrin Beta, Non-erythrocytic 5; N = 2), two genes previously suggested to be associated with SA etiology. Examination of variants with autosomal recessive and X-linked recessive inheritance identified five and two missense variants, respectively. Compound heterozygous variants were identified in several genes. In addition, 12 de novo variants were identified, all in different genes in different children. CONCLUSIONS: To our knowledge, this is the first study reporting a possible association between ID1 and non-syndromic SA. Although maternal pregestational diabetes has been strongly associated with SA, the missense variants in ID1 identified in two of three children were paternally inherited. These findings add to the knowledge of gene variants associated with non-syndromic SA and provide data for future studies.

Modeling complex effects of exposure to particulate matter and extreme heat during pregnancy on congenital heart defects: A U.S. population-based case-control study in the National Birth Defects Prevention Study.

BACKGROUND/OBJECTIVE: Research suggests gestational exposure to particulate matter ≤2.5 µm (PM2.5) and extreme heat may independently increase risk of birth defects. We investigated whether duration of gestational extreme heat exposure modifies associations between PM2.5 exposure and specific congenital heart defects (CHDs). We also explored nonlinear exposure-outcome relationships. METHODS: We identified CHD case children (n = 2824) and non-malformed live-birth control children (n = 4033) from pregnancies ending between 1999 and 2007 in the National Birth Defects Prevention Study, a U.S. population-based multicenter case-control study. We assigned mothers 6-week averages of PM2.5 exposure during the cardiac critical period (postconceptional weeks 3-8) using the closest monitor within 50 km of maternal residence. We assigned a count of extreme heat days (EHDs, days above the 90th percentile of daily maximum temperature for year, season, and weather station) during this period using the closest weather station. Using generalized additive models, we explored logit-nonlinear exposure-outcome relationships, concluding logistic models were reasonable. We estimated joint effects of PM2.5 and EHDs on six CHDs using logistic regression models adjusted for mean dewpoint and maternal age, education, and race/ethnicity. We assessed multiplicative and additive effect modification. RESULTS: Conditional on the highest observed EHD count (15) and at least one critical period day during spring/summer, each 5 µg/m3 increase in average PM2.5 exposure was significantly associated with perimembranous ventricular septal defects (VSDpm; OR: 1.54 [95% CI: 1.01, 2.41]). High EHD counts (8+) in the same population were positively, but non-significantly, associated with both overall septal defects and VSDpm. Null or inverse associations were observed for lower EHD counts. Multiplicative and additive effect modification estimates were consistently positive in all septal models. CONCLUSIONS: Results provide limited evidence that duration of extreme heat exposure modifies the PM2.5-septal defects relationship. Future research with enhanced exposure assessment and modeling techniques could clarify these relationships.