Table 2.

Adjusted associations between unconventional gas development (UGD) activity during pregnancy and preterm birth among 80,257 women with a singleton birth in the Barnett Shale, 30 November 2010–29 November 2012.

Exposure	UGD-Drilling Activity			UGD-Production Activity
	n	Cases	aORa (95% CI)	n	Cases	aORa (95% CI)
0 Wells	68,256	11,290	Reference	68,256	11,290	Reference
1st Tertile	1,813	283	1.03 (0.90, 1.18)	3,502	577	1.07 (0.97, 1.17)
2nd Tertile	1,831	295	1.03 (0.90, 1.18)	3,519	608	1.13 (1.02, 1.24)
3rd Tertile	1,912	342	1.20 (1.06, 1.37)	3,621	635	1.15 (1.05, 1.26)
p-Trendb			$<0.01$			$<0.01$

Note: aOR, adjusted odds ratio; CI, confidence interval.

^aORs were derived using conditional logistic regression, and five controls were individually matched to each case on maternal age ($\le 20$, 21–25, 26–30, 31–35, $\ge 35\mathrm{y}$) and race/ethnicity (non-Hispanic black, Hispanic, non-Hispanic white, other). The time at risk of each control was truncated based on the gestational age of the matched case. ORs were adjusted for prepregnancy body mass index, maternal education, smoking, Adequacy of Prenatal Care Utilization index, previous poor pregnancy outcome, and infant sex.

^bComputed by including the UGD variable in the regression model as a continuous variable.