Table 4.

Adjusted associations between unconventional gas development (UGD) activity during pregnancy and severity of 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)
Extremely preterm
0 Wells	68,256	873	Reference	68,256	873	Reference
1st Tertile	1,813	15	1.00 (0.56, 1.81)	3,502	49	1.22 (0.86, 1.74)
2nd Tertile	1,831	13	0.66 (0.35, 1.22)	3,519	46	1.14 (0.80, 1.63)
3rd Tertile	1,912	28	2.00 (1.23, 3.24)	3,621	38	1.53 (1.03, 2.27)
p-trendb			0.12			0.03
Very preterm
0 Wells	68,256	1,194	Reference	68,256	1,194	Reference
1st Tertile	1,813	31	1.31 (0.87, 1.98)	3,502	48	0.86 (0.62, 1.20)
2nd Tertile	1,831	32	0.95 (0.63, 1.42)	3,519	61	1.19 (0.88, 1.60)
3rd Tertile	1,912	27	0.97 (0.62, 1.52)	3,621	54	1.01 (0.74, 1.39)
p-trendb			0.98			0.67
Moderately preterm
0 Wells	68,256	9,223	Reference	68,256	9,223	Reference
1st Tertile	1,813	237	1.00 (0.86, 1.17)	3,502	480	1.08 (0.97, 1.20)
2nd Tertile	1,831	250	1.06 (0.92, 1.23)	3,519	501	1.12 (1.01, 1.24)
3rd Tertile	1,912	287	1.18 (1.03, 1.36)	3,621	543	1.15 (1.04, 1.27)
p-Trendb			0.02			$<0.01$

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.