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Importance: Buprenorphine combined with naloxone is commonly used to treat opioid use disorders outside of pregnancy. In pregnancy, buprenorphine alone is generally recommended because of limited perinatal safety data on the combination product., Objective: To compare perinatal outcomes following prenatal exposure to buprenorphine with naloxone vs buprenorphine alone., Design, Settings, and Participants: Population-based cohort study using health care utilization data from Medicaid-insured beneficiaries in the US from 2000 to 2018. The cohort was restricted to pregnant individuals linked to their liveborn infants, with maternal Medicaid enrollment from 3 months before pregnancy to 1 month after delivery and infant enrollment for the first 3 months after birth, unless they died sooner., Exposure: Use of buprenorphine with naloxone vs buprenorphine alone during the first trimester based on outpatient dispensings., Main Outcomes and Measures: Outcomes included major congenital malformations, low birth weight, neonatal abstinence syndrome, neonatal intensive care unit admission, preterm birth, respiratory symptoms, small for gestational age, cesarean delivery, and maternal morbidity. Confounder-adjusted risk ratios were calculated using propensity score overlap weights., Results: This study identified 3369 pregnant individuals exposed to buprenorphine with naloxone during the first trimester (mean [SD] age, 28.8 [4.6] years) and 5326 exposed to buprenorphine alone or who switched from the combination to buprenorphine alone by the end of the first trimester (mean [SD] age, 28.3 [4.5] years). When comparing buprenorphine combined with naloxone with buprenorphine alone, a lower risk for neonatal abstinence syndrome (absolute risk, 37.4% vs 55.8%; weighted relative risk, 0.77 [95% CI, 0.70-0.84]) and a modestly lower risk for neonatal intensive care unit admission (absolute risk, 30.6% vs 34.9%; weighted relative risk, 0.91 [95% CI, 0.85-0.98]) and small for gestational age (absolute risk, 10.0% vs 12.4%; weighted relative risk, 0.86 [95% CI, 0.75-0.98]) was observed. For maternal morbidity, the comparative rates were 2.6% vs 2.9%, respectively, and the weighted relative risk was 0.90 (95% CI, 0.68-1.19). No differences were observed with respect to major congenital malformations overall, low birth weight, preterm birth, respiratory symptoms, or cesarean delivery. Results were consistent across sensitivity analyses., Conclusions and Relevance: There were similar and, in some instances, more favorable neonatal and maternal outcomes for pregnancies exposed to buprenorphine combined with naloxone compared with buprenorphine alone. For the outcomes assessed, compared with buprenorphine alone, buprenorphine with naloxone during pregnancy appears to be a safe treatment option. This supports the view that both formulations are reasonable options for the treatment of opioid use disorder in pregnancy, affirming flexibility in collaborative treatment decision-making.

Tyrosine kinase inhibitors (TKI) have improved the outcome and life expectancy of patients with chronic myeloid leukemia (CML). Patients are diagnosed with CML at younger ages, and patients treated for CML may become pregnant or choose to breastfeed. The information available to date on the safety of TKIs during pregnancy and lactation and the optimal management of these patients is largely anecdotal, based on personal or small-group experience, and heterogeneous. A panel of interested parties was convened by U.S. Food and Drug Administration to analyze the current data and discuss possible solutions. Possible solutions include prospective data collection, in clinical trials and in routine clinical practice, a more uniform and specific data collection, and greater coordination among involved entities. As patients with cancer are living longer, frequently receiving therapies for extended periods of time (or for life), data on appropriate management of patients through different reproductive phases of life are needed. It is thus time to change our approach for how to study treatment of cancer (including CML) during pregnancy or breastfeeding to develop evidence-based guidelines for safe and effective patient care., (©2024 American Association for Cancer Research.)

Objective: We emulated a modified randomized trial (Metformin in Women With Type 2 Diabetes in Pregnancy [MiTy]) to compare the perinatal outcomes in women continuing versus discontinuing metformin during pregnancy among those with type 2 diabetes treated with metformin plus insulin before pregnancy., Research Design and Methods: This study used two health care claims databases (U.S., 2000-2020). Pregnant women age 18-45 years with type 2 diabetes who were treated with metformin plus insulin at conception were eligible. The primary outcome was a composite of preterm birth, birth injury, neonatal respiratory distress, neonatal hypoglycemia, and neonatal intensive care unit admission. Secondary outcomes included the components of the primary composite outcome, gestational hypertension, preeclampsia, maternal hypoglycemia, cesarean delivery, infants large for gestational age, infants small for gestational age (SGA), sepsis, and hyperbilirubinemia. We adjusted for potential baseline confounders, including demographic characteristics, comorbidities, and proxies for diabetes progression., Results: Of 2,983 eligible patients, 72% discontinued use of metformin during pregnancy. The average age at conception was 32 years, and the prevalence of several comorbidities was higher among continuers. The risk of the composite outcome was 46% for continuers and 48% for discontinuers. The adjusted risk ratio was 0.92 (95% CI 0.81, 1.03). Risks were similar between treatments and consistent between databases for most secondary outcomes, except for SGA, which was elevated in continuers only in the commercially insured population., Conclusions: Our findings were consistent with those reported in the MiTy randomized trial. Continuing metformin during pregnancy was not associated with increased risk of a neonatal composite adverse outcome. However, a possible metformin-associated risk of SGA warrants further consideration., (© 2024 by the American Diabetes Association.)

Background: Although the clinical importance of preeclampsia is widely recognized, few treatment options are available for prevention. TNF-α inhibitors have been hypothesized to potentially prevent the disease. We aimed to examine whether exposure to TNF-α inhibitors during pregnancy reduces the risk of preeclampsia., Methods: We conducted a population-based pregnancy cohort study using nationwide samples of publicly (Medicaid data, 2000-2018) and commercially (MarketScan Research Database, 2003-2020) insured pregnant women linked to their liveborn infants. Exposure was ascertained based on a filled prescription or administration code for TNF-α inhibitors during the first and second trimester of pregnancy. The outcomes included early-onset preeclampsia, late-onset preeclampsia, and small-for-gestational age. For baseline confounding adjustment, we leveraged propensity score overlap weights to estimate risk ratios (RR)., Results: Among 4 315 658 pregnancies in the Medicaid and the MarketScan cohort, 2736 (0.063%) were exposed to TNF-α inhibitors during the first trimester and 1712 (0.040%) during the second trimester. After adjustment, the risk of early-onset preeclampsia was not decreased among mothers exposed during the first trimester compared with unexposed women with treatment indications [RR pooled : 1.25, 95% confidence interval (CI) 0.93-1.67]. Similarly, the risk of late-onset preeclampsia was not decreased among mothers exposed during the second trimester compared with unexposed women (RR pooled : 0.99, 95% CI 0.81-1.22)., Conclusion: Contrary to the hypothesis, exposure to TNF-α inhibitors during pregnancy did not appear to be associated with a reduced risk of early-onset or late-onset preeclampsia. These findings do not support consideration of the use of TNF-α inhibitors for the prevention of preeclampsia., (Copyright © 2024 Wolters Kluwer Health, Inc. All rights reserved.)

Fertility procedures recorded in health-care databases can be used to estimate the start of pregnancy, which can serve as a reference standard to validate gestational age estimates based on International Classification of Diseases codes. In a cohort of 17 398 US MarketScan pregnancies (2011-2020) in which conception was achieved via fertility procedures, we estimated gestational age at the end of pregnancy using algorithms based on (1) time (days) since the fertility procedure (the reference standard); (2) International Classification of Diseases, Ninth Revision (ICD-9)/International Classification of Diseases, Tenth Revision (ICD-10) (before/after October 2015) codes indicating gestational length recorded at the end of pregnancy (method A); and (3) ICD-10 end-of-pregnancy codes enhanced with Z3A codes denoting specific gestation weeks recorded at prenatal visits (method B). We calculated the proportion of pregnancies with an estimated gestational age falling within 14 days ($\pm$14 days) of the reference standard. Method A accuracy was similar for ICD-9 and ICD-10 codes. After 2015, method B was more accurate than method A: For term births, within-14-day agreement was 90.8% for method A and 98.7% for method B. Corresponding estimates were 70.1% and 95.6% for preterm births; 35.3% and 92.6% for stillbirths; 54.3% and 64.2% for spontaneous abortions; and 16.7% and 84.6% for elective terminations. ICD-10-based algorithms that incorporate Z3A codes improve the accuracy of gestational age estimation in health-care databases, especially for preterm births and non-live births., (© The Author(s) 2024. Published by Oxford University Press on behalf of the Johns Hopkins Bloomberg School of Public Health. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Background: Past research on the safety of prenatal exposure to medications has focused on maternal use during gestation, with limited research into the potential effects of paternal use during the spermatogenic period preceding conception. Knowing the most common medications used by fathers around the time of conception can inform research priorities in this field., Objectives: To identify the most common medications dispensed to fathers in the preconception period., Methods: Within the MarketScan research database of commercially insured individuals in the United States from 2011 to 2020, we identified pregnancies, estimated the date of conception, linked each pregnancy to the father using family enrolment information and required minimum enrolment period and prescription benefits. Then, we described the use of prescription medications by the father during the 90 days before conception based on pharmacy dispensation claims., Results: Of 4,437,550 pregnancies, 51.6% were linked with a father. Among the 1,413,762 pregnancies connected with a father that also met the inclusion criteria, the most common classes of medications dispensed were psychotropics (8.66%), antibiotics (7.21%), and analgesics (6.82%). The most frequently dispensed medications were amoxicillin (3.75%), azithromycin (3.15%), fluticasone (2.70%) and acetaminophen/hydrocodone (2.70%). Some fathers filled prescriptions for medications associated with foetal embryopathy when used by the mother, including mycophenolate (0.04%), methotrexate (0.03%) and isotretinoin (0.02%)., Conclusions: More than a third of fathers filled at least one prescription medication in the preconception period, and several of them are known to be embryotoxic, emphasizing the necessity for further investigation into the potential teratogenicity of paternal exposure., (© 2024 John Wiley & Sons Ltd.)

Background: Medication use during pregnancy has increased in the United States despite the lack of safety data for many medications., Objective: This study aimed to inform research priorities by examining trends in medication use during pregnancy and identifying gaps in safety information on the most commonly prescribed medications., Study Design: We identified population-based cohorts of commercially (MarketScan 2011-2020) and publicly (Medicaid Analytic eXtract/Transformed Medicaid Statistical Information System Analytic Files 2011-2018) insured pregnancies ending in live birth from 2 health care utilization databases. Medication use was based on filled prescriptions between the date of last menstrual period through delivery, as well as the period before the last menstrual period and during specific trimesters. We also included a cross-sectional representative sample of pregnancies ascertained by the National Health and Nutrition Examination Survey (2011-2020), with information on prescription medication use during the preceding month obtained through maternal interviews. Teratogen Information System was used to classify the available evidence on teratogenic risk., Results: Among over 3 million pregnancies, the medications most commonly dispensed at any time during pregnancy were analgesics, antibiotics, and antiemetics. The top medications were ondansetron (16.8%), amoxicillin (13.5%), and azithromycin (12.4%) in MarketScan, nitrofurantoin (22.2%), acetaminophen (21.3%; mostly as part of acetaminophen-hydrocodone products), and ondansetron (19.5%) in Medicaid Analytic eXtract/Transformed Medicaid Statistical Information System Analytic Files, and levothyroxine (5.0%), sertraline (2.9%), and insulin (2.9%) in the National Health and Nutrition Examination Survey group. The most commonly dispensed suspected teratogens during the first trimester were antithyroid medications. The use of antidiabetic and psychotropic medications has continued to increase in the United States during the last decade, opioid dispensation has decreased by half, and antibiotics and antiemetics continue to be common. For one-quarter of medications, there is insufficient evidence available to characterize their safety profile in pregnancy., Conclusion: There is a need for more drug research in pregnant patients. Future research should focus on anti-infectives with high utilization and limited level of evidence on safety for use during pregnancy. Although lack of evidence is not evidence of safety concerns, it does not indicate risk either. In many instances, the benefits outweigh the risks when these medications are used clinically, and some of the medications with no proven safety may be necessary to treat patients., (Copyright © 2023 Elsevier Inc. All rights reserved.)

Background: Metformin is the most used oral antidiabetic medication. Despite its established safety profile, it has known antiandrogenic and epigenetic modifying effects. This raised concerns about possible adverse developmental effects caused by genomic alterations related to paternal use of metformin during the spermatogenesis period preceding conception., Objective: To assess the potential adverse intergenerational effect of metformin by examining the association between paternal metformin use during spermatogenesis and major congenital malformations (MCMs) in newborns., Design: Nationally representative cohort study., Setting: A large Israeli health fund., Participants: 383 851 live births linked to fathers and mothers that occurred in 1999 to 2020., Measurements: MCMs and parental cardiometabolic conditions were ascertained using clinical diagnoses, medication dispensing information, and laboratory test results. The effect of metformin use on MCMs was estimated using general estimating equations, accounting for concurrent use of other antidiabetic medications and parental cardiometabolic morbidity., Results: Compared with unexposed fathers, the prevalence of cardiometabolic morbidity was substantially higher among fathers who used metformin during spermatogenesis, and their spouses. Whereas the crude odds ratio (OR) for paternal metformin exposure in all formulations and MCMs was 1.28 (95% CI, 1.01 to 1.64), the adjusted OR was 1.00 (CI, 0.76 to 1.31). Within specific treatment regimens, the adjusted OR was 0.86 (CI, 0.60 to 1.23) for metformin in monotherapy and 1.36 (CI, 1.00 to 1.85) for metformin in polytherapy, a treatment that was more common in patients with more poorly controlled diabetes., Limitation: Laboratory test results for hemoglobin A 1c to assess underlying diabetes severity were available only for a subset of the cohort., Conclusion: Paternal use of metformin in monotherapy does not increase the risk for MCMs. Association for metformin in polytherapy could potentially be explained by worse underlying parental cardiometabolic risk profile., Primary Funding Source: None., Competing Interests: Disclosures: Disclosures can be viewed at www.acponline.org/authors/icmje/ConflictOfInterestForms.do?msNum=M23-1405.

Background: Metformin is a first-line pharmacotherapy for type 2 diabetes, but there is limited evidence about its safety in early pregnancy., Objective: To evaluate the teratogenicity of metformin use in the first trimester of pregnancy., Design: In an observational cohort of pregnant women with pregestational type 2 diabetes receiving metformin monotherapy before the last menstrual period (LMP), a target trial with 2 treatment strategies was emulated: insulin monotherapy (discontinue metformin treatment and initiate insulin within 90 days of LMP) or insulin plus metformin (continue metformin and initiate insulin within 90 days of LMP)., Setting: U.S. Medicaid health care administration database (2000 to 2018)., Participants: 12 489 pregnant women who met the eligibility criteria., Measurements: The risk and risk ratio of nonlive births, live births with congenital malformations, and congenital malformations among live births were estimated using standardization to adjust for covariates., Results: A total of 850 women were in the insulin monotherapy group and 1557 in the insulin plus metformin group. The estimated risk for nonlive birth was 32.7% under insulin monotherapy (reference) and 34.3% under insulin plus metformin (risk ratio, 1.02 [95% CI, 1.01 to 1.04]). The estimated risk for live birth with congenital malformations was 8.0% (CI, 5.7% to 10.2%) under insulin monotherapy and 5.7% (CI, 4.5% to 7.3%) under insulin plus metformin (risk ratio, 0.72 [CI, 0.51 to 1.09])., Limitation: Possible residual confounding by glycemic control and body mass index., Conclusion: Compared with switching to insulin monotherapy, continuing metformin and adding insulin in early pregnancy resulted in little to no increased risk for nonlive birth among women receiving metformin before pregnancy. Under conventional statistical criteria, anything between a 49% decrease and a 9% increase in risk for congenital malformations was highly compatible with our data., Primary Funding Source: National Institutes of Health., Competing Interests: Disclosures: Disclosures can be viewed at www.acponline.org/authors/icmje/ConflictOfInterestForms.do?msNum=M23-2038.

Competing Interests: The authors report no conflicts of interest.

Background: Maternal use of valproate during pregnancy has been associated with an increased risk of neurodevelopmental disorders in children. Although most studies of other antiseizure medications have not shown increased risks of these disorders, there are limited and conflicting data regarding the risk of autism spectrum disorder associated with maternal topiramate use., Methods: We identified a population-based cohort of pregnant women and their children within two health care utilization databases in the United States, with data from 2000 through 2020. Exposure to specific antiseizure medications was defined on the basis of prescription fills from gestational week 19 until delivery. Children who had been exposed to topiramate during the second half of pregnancy were compared with those unexposed to any antiseizure medication during pregnancy with respect to the risk of autism spectrum disorder. Valproate was used as a positive control, and lamotrigine was used as a negative control., Results: The estimated cumulative incidence of autism spectrum disorder at 8 years of age was 1.9% for the full population of children who had not been exposed to antiseizure medication (4,199,796 children). With restriction to children born to mothers with epilepsy, the incidence was 4.2% with no exposure to antiseizure medication (8815 children), 6.2% with exposure to topiramate (1030 children), 10.5% with exposure to valproate (800 children), and 4.1% with exposure to lamotrigine (4205 children). Propensity score-adjusted hazard ratios in a comparison with no exposure to antiseizure medication were 0.96 (95% confidence interval [CI], 0.56 to 1.65) for exposure to topiramate, 2.67 (95% CI, 1.69 to 4.20) for exposure to valproate, and 1.00 (95% CI, 0.69 to 1.46) for exposure to lamotrigine., Conclusions: The incidence of autism spectrum disorder was higher among children prenatally exposed to the studied antiseizure medications than in the general population. However, after adjustment for indication and other confounders, the association was substantially attenuated for topiramate and lamotrigine, whereas an increased risk remained for valproate. (Funded by the National Institute of Mental Health.)., (Copyright © 2024 Massachusetts Medical Society.)

Importance: Use of buprenorphine or methadone to treat opioid use disorder is recommended in pregnancy; however, their teratogenic potential is largely unknown., Objective: To compare the risk of congenital malformations following in utero exposure to buprenorphine vs methadone., Design, Setting, and Participants: This population-based cohort study used health care utilization data from publicly insured Medicaid beneficiaries in the US from 2000 to 2018. A total of 13 360 pregnancies with enrollment from 90 days prior to pregnancy start through 1 month after delivery and first trimester use of buprenorphine or methadone were included and linked to infants. Data were analyzed from July to December 2022., Exposure: A pharmacy dispensing of buprenorphine or a code for administration of methadone in the first trimester., Main Outcomes and Measures: Primary outcomes included major malformations overall and malformations previously associated with opioids (any cardiac malformations, ventricular septal defect, secundum atrial septal defect/nonprematurity-related patent foramen ovale, neural tube defects, clubfoot, and oral clefts). Secondary outcomes included other organ system-specific malformations. Risk differences and risk ratios (RRs) were estimated comparing buprenorphine with methadone, adjusting for confounders with propensity score overlap weights., Results: The cohort included 9514 pregnancies with first-trimester buprenorphine exposure (mean [SD] maternal age, 28.4 [4.6] years) and 3846 with methadone exposure (mean [SD] maternal age, 28.8 [4.7] years). The risk of malformations overall was 50.9 (95% CI, 46.5-55.3) per 1000 pregnancies for buprenorphine and 60.6 (95% CI, 53.0-68.1) per 1000 pregnancies for methadone. After confounding adjustment, buprenorphine was associated with a lower risk of malformations compared with methadone (RR, 0.82; 95% CI, 0.69-0.97). Risk was lower with buprenorphine for cardiac malformations (RR, 0.63; 95% CI, 0.47-0.85), including both ventricular septal defect (RR, 0.62; 95% CI, 0.39-0.98) and secundum atrial septal defect/nonprematurity-related patent foramen ovale (RR, 0.54; 95% CI, 0.30-0.97), oral clefts (RR, 0.65; 95% CI, 0.35-1.19), and clubfoot (RR, 0.55; 95% CI, 0.32-0.94). Results for neural tube defects were uncertain given low event counts. In secondary analyses, buprenorphine was associated with a decreased risk of central nervous system, urinary, and limb malformations but a greater risk of gastrointestinal malformations compared with methadone. These findings were consistent in sensitivity and bias analyses., Conclusions and Relevance: In this cohort study, the risk of most malformations previously associated with opioid exposure was lower in buprenorphine-exposed infants compared with methadone-exposed infants, independent of measured confounders. Malformation risk is one factor that informs the individualized patient decision regarding medications for opioid use disorder in pregnancy.

Background and Objective: Over the past 2 years, several drugs have been approved for coronavirus disease 2019 (COVID-19) treatment, but their safety during pregnancy remains poorly understood. This study aims to assess the relative risk of obstetric, neonatal, and infant outcomes associated with the use of drugs specifically indicated for the treatment of COVID-19 compared with other drug treatment strategies. The purpose of this article is to present elements of the study protocol., Methods: The COVID-19 International Drug Pregnancy Registry (COVID-PR) is a noninterventional, postmarketing cohort study. Pregnant women receiving treatment with monoclonal antibodies (mAbs) or antiviral drugs for mild, moderate, or severe COVID-19 are matched 1:1 with pregnant women not receiving these study-specific drugs, based on calendar time, country, gestational age at enrollment, and COVID-19 severity. Participants complete online questionnaires at enrollment, during pregnancy, and for 12 months after delivery of liveborn infants. The study began enrolling participants on 1 December 2021 and is set to span 5 years for each drug of interest., Discussion: The COVID-PR is designed to evaluate the safety profile of each studied drug. Additionally, it may allow for an analysis of the effects of COVID-19 drug exposure during relevant gestational periods on specific neonatal outcomes. Although the sample size will be too small to detect associations with rare outcomes, the study has the potential to generate hypotheses for future research. Ultimately, these data can provide valuable insights for evidence-based decisions about COVID-19 treatment during pregnancy., Trial Registration: ClinicalTrials.gov: NCT05013632. EU PAS EUPAS42517., (© 2023. The Author(s), under exclusive licence to Springer Nature Switzerland AG.)

Importance: Increasing use of second-line noninsulin antidiabetic medication (ADM) in pregnant individuals with type 2 diabetes (T2D) may result in fetal exposure, but their teratogenic risk is unknown., Objective: To evaluate periconceptional use of second-line noninsulin ADMs and whether it is associated with increased risk of major congenital malformations (MCMs) in the infant., Design, Setting, and Participants: This observational population-based cohort study used data from 4 Nordic countries (2009-2020), the US MarketScan Database (2012-2021), and the Israeli Maccabi Health Services database (2009-2020). Pregnant women with T2D were identified and their live-born infants were followed until up to 1 year after birth., Exposure: Periconceptional exposure was defined as 1 or more prescription fill of sulfonylureas, dipeptidyl peptidase 4 (DPP-4) inhibitors, glucagon-like peptide 1 (GLP-1) receptor agonists, and sodium-glucose cotransporter 2 (SGLT2) inhibitors, or insulin (active comparator) from 90 days before pregnancy to end of first trimester., Main Outcomes and Measures: Relative risks (RRs) and 95% CIs for MCMs were estimated using log-binomial regression models, adjusting for key confounders in each cohort and meta-analyzed., Results: Periconceptional exposure to second-line noninsulin ADMs differed between countries (32, 295, and 73 per 100 000 pregnancies in the Nordics, US, and Israel, respectively), and increased over the study period, especially in the US. The standardized prevalence of MCMs was 3.7% in all infants (n = 3 514 865), 5.3% in the infants born to women with T2D (n = 51 826), and among infants exposed to sulfonylureas was 9.7% (n = 1362); DPP-4 inhibitors, 6.1% (n = 687); GLP-1 receptor agonists, 8.3% (n = 938); SGLT2 inhibitors, 7.0% (n = 335); and insulin, 7.8% (n = 5078). Compared with insulin, adjusted RRs for MCMs were 1.18 (95% CI, 0.94-1.48), 0.83 (95% CI, 0.64-1.06), 0.95 (95% CI, 0.72-1.26), and 0.98 (95% CI, 0.65-1.46) for infants exposed to sulfonylureas, DPP-4 inhibitors, GLP-1 receptor agonists, and SGLT2 inhibitors, respectively., Conclusions and Relevance: Use of second-line noninsulin ADMs is rapidly increasing for treatment of T2D and other indications, resulting in an increasing number of exposed pregnancies. Although some estimates were imprecise, results did not indicate a large increased risk of MCMs above the risk conferred by maternal T2D requiring second-line treatment. Although reassuring, confirmation from other studies is needed, and continuous monitoring will provide more precise estimates as data accumulate.

Associations between gaseous pollutant exposure and stillbirth have focused on exposures averaged over trimesters or gestation. We investigated the association between short-term increases in nitrogen dioxide (NO 2 ) and ozone (O 3 ) concentrations and stillbirth risk among a national sample of 116 788 Medicaid enrollees from 2000 to 2014. A time-stratified case-crossover design was used to estimate distributed (lag 0-lag 6) and cumulative lag effects, which were adjusted for PM 2.5 concentration and temperature. Effect modification by race/ethnicity and proximity to hydraulic fracturing (fracking) wells was assessed. Short-term increases in the NO 2 and O 3 concentrations were not associated with stillbirth in the overall sample. Among American Indian individuals ( n = 1694), a 10 ppb increase in NO 2 concentrations was associated with increased stillbirth odds at lag 0 (5.66%, 95%CI: [0.57%, 11.01%], p = 0.03) and lag 1 (4.08%, 95%CI: [0.22%, 8.09%], p = 0.04) but not lag 0-6 (7.12%, 95%CI: [-9.83%, 27.27%], p = 0.43). Among participants living in zip codes within 15 km of active fracking wells ( n = 9486), a 10 ppb increase in NO 2 concentration was associated with increased stillbirth odds in single-day lags (2.42%, 95%CI: [0.37%, 4.52%], p = 0.02 for lag 0 and 1.83%, 95%CI: [0.25%, 3.43%], p = 0.03 for lag 1) but not the cumulative lag (lag 0-6) (4.62%, 95%CI: [-2.75%, 12.55%], p = 0.22). Odds ratios were close to the null in zip codes distant from fracking wells. Future studies should investigate the role of air pollutants emitted from fracking and potential racial disparities in the relationship between short-term increases in NO 2 concentrations and stillbirth.