Table 3.

Adjusted mean differences in gestational age for a +10-percentage point increase in the probability that private well arsenic concentrations exceed the respective threshold, overall and stratified by county-level rates of private well use.

Gestational Age (weeks) β (95% CI)
Private well arsenic level	Overall	0–25% private well use	>25–50% private well use	>50–75% private well use	>75% private well use
No. of Births	3,580,755	3,045,265	426,397	98,745	10,348
No. of Counties	3,105	1,522	1,091	415	76
Boosted regression tree: 1 μg/L
≤1 μg/L	0.00 (reference)	0.00 (reference)	0.00 (reference)	0.00 (reference)	0.00 (reference)
>1 μg/L	−0.01 (−0.01, −0.01)*	−0.01 (−0.01, −0.01)*	−0.01 (−0.02, −0.01)*	0.00 (−0.02, 0.01)	0.04 (−0.01, 0.09)
Boosted regression tree: 5 μg/L
≤5 μg/L	0.00 (reference)	0.00 (reference)	0.00 (reference)	0.00 (reference)	0.00 (reference)
>5 μg/L	−0.01 (−0.01, 0.00)	−0.01 (−0.02, −0.01)*	−0.01 (−0.02, 0.01)	0.00 (−0.02, 0.02)	0.12 (0.02, 0.21)*
Boosted regression tree: 10 μg/L
≤10 μg/L	0.00 (reference)	0.00 (reference)	0.00 (reference)	0.00 (reference)	0.00 (reference)
>10 μg/L	−0.01 (−0.02, 0.00)	−0.02 (−0.03, −0.01)*	0.00 (−0.02, 0.01)	0.01 (−0.02, 0.04)	0.33 (0.15, 0.52)*
Random forest classification: 5 and 10 μg/L
≤5 μg/L	0.00 (reference)	0.00 (reference)	0.00 (reference)	0.00 (reference)	0.00 (reference)
>5 to ≤10 μg/L	0.00 (−0.02, 0.01)	0.00 (−0.02, 0.02)	−0.02 (−0.04, 0.01)	0.00 (−0.06, 0.05)	−0.09 (−0.25, 0.07)
>10 μg/L	0.00 (−0.01, 0.01)	−0.01 (−0.03, 0.00)	0.00 (−0.01, 0.02)	0.01 (−0.02, 0.05)	0.29 (0.15, 0.43)*

Note: CI, confidence interval. Asterisks denote statistical significance. Estimates are adjusted for maternal age, race/ethnicity, marital status, educational attainment, smoking during pregnancy, pre-pregnancy BMI, rurality/urbanicity, and the average annual concentration of particulate matter with aerodynamic diameter less than 2.5 μm in the county with nested random intercepts for maternal county and state of residence.