Table 3.

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)
Trimester One
0 Wells	68,256	11,290	Reference	68,256	11,290	Reference
1st Tertile	1,482	227	1.03 (0.88, 1.19)	4,937	791	1.03 (0.95, 1.12)
2nd Tertile	1,116	169	0.96 (0.80, 1.14)	3,081	558	1.20 (1.09, 1.33)
3rd Tertile	898	167	1.24 (1.03, 1.49)	1,408	252	1.18 (1.02, 1.37)
p-trendb			0.11			$<0.01$
Trimester Two
0 Wells	68,256	11,290	Reference	68,256	11,290	Reference
1st Tertile	1,380	233	1.10 (0.94, 1.28)	5,135	820	1.02 (0.94, 1.11)
2nd Tertile	1,077	183	1.15 (0.96, 1.33)	3,324	594	1.19 (1.08, 1.31)
3rd Tertile	878	161	1.21 (1.00, 1.45)	1,574	279	1.14 (0.99, 1.31)
p-trendb			$<0.01$			$<0.01$
Trimester Three
0 Wells	68,256	11,290	Reference	68,256	11,290	Reference
1st Tertile	799	134	1.13 (0.93, 1.39)	5,994	1,024	1.10 (1.02, 1.19)
2nd Tertile	527	82	0.95 (0.73, 1.22)	2,508	451	1.19 (1.06, 1.33)
3rd Tertile	358	63	1.19 (0.89, 1.60)	745	109	0.89 (0.72, 1.10)
p-Trendb			0.31			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.