Each PUR record includes the name of the pesticide’s active ingredient, the poundage applied, the crop type, and the location and the date of application. The California Department of Water Resources performs countywide, large-scale surveys of land use and crop cover every 7–10 years. Land use maps increase spatial resolution because they provide more detailed land use geography that allows us to refine the pesticide applications . We then combined PUR records, land use maps, and geocoded birth addresses to produce estimates of pesticide exposure during pregnancy. Monthly exposure estimates were calculated by adding the poundage of pesticide applied in a 2-kilometer buffer surrounding each address and weighting the total poundage by the proportion of acreage treated within the buffer. Previous pesticide studies relied on different buffer sizes from 500m , half a mile , 1000m , 1250m , 1600m , 5000m , to up to 8000m distances , depending on the pesticide of interest, landscape, and weather conditions. In light of previous research, the buffer of 2-km we chose, will provide a reasonable distance for assessing pesticide applications around residential addresses. For each calendar month, our integrated GIS-system returned continuous measures for each specific chemical applied within 2-km of individuals’ residences. We defined the first, second, and third trimesters as 0-12 weeks, 13-25 weeks, and ≥26 weeks of pregnancy, respectively. For preterm birth, the length of gestation and hence exposure period are shorter than term birth by design; to account for that, we assessed the third trimester exposures using 27- 32 weeks of gestation only since more than 88% of all preterm births had a gestational length longer than 32 weeks. For each pesticide, daily poundage for each gestational day of pregnancy was calculated based on monthly values,seedling starter pot and then averaged across all days in each trimester. We then categorized prenatal exposure as ever/never exposed to a specific chemical in each trimester.
We selected 17 individual chemicals previously observed to have reproductive toxicity . Additionally, we also considered all pesticides from three widely used chemical classes that have been linked to reproductive toxicity based on the Pesticide Action Network pesticide database , i.e. 24 n-methyl carbamate/dithiocarbamates, 50 organophosphates, and 29 pyrethroid pesticides to which one or more study subjects were exposed according to our 2km buffer criterion . For each class, we used the sum of the total number of individual chemicals that each subject was ever exposed to in each time period of interest. We divided subjects into high , low , and no exposure to the respective pesticide, and compared high and low with the no exposure group as the reference. Since information about the specific location of non-agricultural pesticide applications are not provided by the PUR and because some individuals in urban areas are highly exposed to traffic-related air pollution or hazardous air toxics that are known risk factors for adverse birth outcomes , we restricted our analyses to individuals born in agricultural regions, defining those as residences within 2km buffer of any type of agricultural pesticide application during pregnancy . We conducted unconditional logistic regression analyses adjusting for matching factors and the source of control subjects and estimated odds ratios and 95% confidence intervals . To account for the unbalanced gender ratio and birth year distribution in this combined sample, we included the inverse of the sampling fraction as a stabilized weighting factor to reflect the sex and birth year distribution of all California births. Statistical analyses were performed using SAS software, Version 9.4 . We additionally adjusted for covariates as potential confounders and effect measure modifiers based on the literature : including maternal age at delivery , maternal race/ethnicity , maternal birthplace , maternal education , parity , payment source for prenatal care as a proxy for family income , prenatal care in the first trimester , and a previously developed neighborhood-level SES metric . Furthermore, we conducted stratified analyses by maternal race/ethnicity since exposures may be higher among Hispanics, especially recent immigrants, who may live close to agricultural fields and have poor housing conditions ; by infant sex because males are more likely to be born preterm ; as well as by season of conception , estimated from the last menstrual period and length of gestation, because of seasonal variations in pesticide applications .
In sensitivity analyses, we compared effect estimates with and without adjusting for two risk factors for adverse birth outcomes, maternal cigarette smoking during pregnancy and prepregnancy Body Mass Index , calculated as maternal pre-pregnancy weight divided by maternal height for births in 2007-2010 only, since these variables are only available on the birth certificate from 2007 onward. We also investigated the potential confounding effects from outdoor air pollution that can impact fetal growth during critical periods among the autism controls only due to data availability. We estimated trimester-specific exposures to local, traffic-derived NOx, PM2.5, and CO, including roadways within 1.5 km of subjects’ birth addresses, i.e. inter-quartile range -scaled measure of NOx as a local traffic marker derived from the CAlifornia LINE source dispersion model model . Additionally, we adjusted for co-exposure to at least one of other individual chemicals as a single variable when assessing each individual chemical, and estimated mutually adjusted ORs for the three chemical class exposures during the same exposure window. When evaluating later trimester exposures we adjusted for exposure during prior pregnancy periods, because these effect estimates may be altered by earlier exposures . Since a low geocode quality is likely to introduce spatial exposure misclassification, we excluded those with a geocode quality at the USPS Zip Code Area centroid level or coarser. Lastly, we examined spontaneous vaginal deliveries only, excluding medically indicated preterm deliveries more likely to be due to severe maternal pregnancy complications including pre-eclampsia and gestational diabetes that might or might not be in the causal pathway for pesticide exposures and the outcome. Infants born preterm or born term with low birthweight were more likely to have mothers of younger age, less education, lower neighborhood SES, starting prenatal care after the first trimester, and using Medi-Cal or other government programs instead of private insurance. In addition, infants born preterm were more likely to be a third or later born child, and have mothers with Hispanic or Black race/ethnic background; infants born term but with low birthweight were more likely to be female and a first born child, and born to Black and Asian mothers . First- and second trimester exposures to some pesticides we have selected were associated with a small increase in risk for preterm birth. Specifically, in multivariate adjusted models, first trimester exposures to glyphosate compounds, paraquat dichloride, chlorpyrifos, imidacloprid, permethrin, dimethoate, and methyl bromide, and second trimester exposures to chlorothalonil, glyphosate compounds, paraquat dichloride, simazine, and imidacloprid, yielded adjusted ORs between 1.03 and 1.07 with 95% CIs excluding the null value .
Maternal education changed the OR estimates the most among all covariates.Effect estimates were generally slightly stronger in female infants, except for simazine, which shows stronger effect in males with an OR of 1.06~1.07 . Stratified analysis by season of conception suggested that effect estimates were generally stronger when the peak season of pesticide application concurred with the first or second trimester of pregnancy . When examining chemical classes,round nursery pots first trimester exposures to carbamates , or pyrethroids increased ORs for preterm birth in the high exposure group, compared with the no exposure group, while second trimester exposures to carbamates, organophosphates, or pyrethroids were all associated with small increases in ORs for preterm birth . We generally did not observe elevated ORs for preterm birth among male infants, but observed a stronger 7–11% increase with exposure during the first or second trimester among female infants . Exposure prevalence and effect estimates were generally stronger in infants born to the foreign born or US-born Hispanic mothers than White mothers . Associations between the selected individual pesticides or chemical classes and term low birthweight for each trimester in pregnancy were mostly null. In multivariate adjusted models, we only saw increased ORs for second or third-trimester exposures to myclobutanil ; similarly, exposures to the three chemical classes were not associated with term low birthweight in general, except for marginally elevated odds in infants exposed to 2 or more pyrethroids . Results were similar in our sensitivity analyses, with additional adjustment for maternal prepregnancy BMI and maternal smoking in the years 2007-2010, for NOx as traffic-related air pollution, or restricting to those with a high geocode quality only. For each individual pesticide, adjusting for co-exposure to other pesticides resulted in attenuation of odds by 2-3%; ORs mutually adjusted of three chemical classes or adjusted for prior exposures were mostly similar to or slightly decreased; the mutually adjusted OR for pyrethroids was most stable, suggesting a more robust association with pyrethroids, which were used more in recent years . ORs were generally stronger when we restricted to spontaneous preterm births only for both individual chemicals and chemical classes. In this large California study of women living within 2km distance from agricultural fields on which pesticides were applied, we found that early and mid-pregnancy exposure to selected pesticides known or suspected to be reproductive toxicants and chemicals in the classes of pyrethroids and possibly also carbamates or organophosphates, are associated with a small to moderate size increase in risk of preterm birth between 1998 and 2010.
We found little evidence for pesticides being related to term low birthweight, except for exposures to pyrethroids as a class further corroborating their adverse influence on pregnancy observed for preterm birth and possibly one single pesticide myclobutanil – however, this might have been a chance observation given that we tested 17 individual chemicals. Yet, term low birth weight is a much rarer event than preterm birth and we had less statistical power to estimate small effects accurately. Our positive findings for preterm birth are consistent with biomarker-based studies with measured organophosphates, or pyrethroids and their metabolic breakdown products in maternal blood or urine or umbilical cord blood , though most of the literature assessing environmental exposures to pesticide found inadequate evidence for associations with preterm birth . Less than a handful of studies conducted in the US examined associations for environmental exposures to pesticides from agricultural applications and preterm birth and/or low birthweight and provided month- or trimester-specific estimates . These studies were almost exclusively conducted using California’s unique PUR system, nevertheless they differed in terms of how they assessed exposures and pregnancy outcomes. Our study was in line with an earlier study in the San Joaquin Valley that assessed pesticides labeled with EPA signal word toxicity by summing up their active ingredients applied in the 2.6 km2 section surrounding maternal residences and reported high exposure to pesticides increased risks of preterm birth and low birthweight by 5-9% overall . In contrast, one study reported mostly negative associations between spontaneous preterm deliveries and exposure to 69 chemical groups or 543 specific chemicals in 1998-2011 in the San Joaquin Valley , perhaps because this study focused on late pregnancy instead of early or mid-pregnancy, which is believed to be the critical period for exposures causing preterm birth , and in addition a ‘live-birth selection bias’ could in part explain the negative effect. The other study in northern California reported methyl bromide use within 5 km of mother’s home was also positively associated with gestational age in the first trimester; yet their results were sensitive to buffer size and could potentially be confounded by chloropicrin or diazinon, often used conjunctively with methyl bromide . Maternal, placental, and fetal factors are thought to determine risk of preterm birth and may be affected by prenatal exposure to environmental chemicals . For example, it is known that chlorpyrifos can cross the placenta and enter the fetus, possibly altering the growth and development of the fetus . Mechanisms by which pesticides may affect risks of preterm birth include interference with immune pathways and inflammation , or with metabolic and endocrine regulatory pathways as well as oxidative stress . For example, in-vitro study results suggested that phosmet and chlorpyrifos alter cell viability and induce an inflammatory cytokine profile, indicating that organophosphates may adversely affect trophoblast cells .