Table 1.

Neonate, maternal, and area-level characteristics of births by oil and gas well production volume category, California 2006–2015. Prepregnancy BMI and smoking during pregnancy were available for 2007–2015 births (2006 births excluded from the missing category).

Variable	$n$ (%)	Production volume			p-Valuea
		No BOE	1–100 BOE/day	GT 100 BOE/day
		($n=\mathrm{2,866,735}$)	($n\mathrm{=}\mathrm{70,615}$)	($n\mathrm{=}\mathrm{50,079}$)
Neonate characteristics
Mean birth weight [g (mean ± SD)]	2,987,429 (100)	$\mathrm{3,327}\pm 528$	$\mathrm{3,318}\pm 527$	$\mathrm{3,316}\pm 527$	$<0.0001$
Mean gestational age [weeks (mean ± SD)]	2,987,429 (100)	$39\pm 2$	$39\pm 2$	$39\pm 2$	$<0.0001$
Sex
Female	1,456,548 (49)	49	48	49	0.2879
Male	1,530,866 (51)	51	52	51	—
Missingb	15 ($<1$)	100	0	0	—
Birth month
January	244,433 (8)	8	8	8	0.3261
February	224,691 (8)	8	8	8	—
March	245,683 (8)	8	8	8	—
April	233,297 (8)	8	8	8	—
May	242,652 (8)	8	8	8	—
June	241,962 (8)	8	8	8	—
July	260,028 (9)	9	9	9	—
August	269,714 (9)	9	9	9	—
September	266,586 (9)	9	9	9	—
October	261,399 (9)	9	9	9	—
November	245,566 (8)	8	8	8	—
December	251,418 (8)	8	8	8	—
Birth year
2006	320,330 (11)	11	10	12	$<0.0001$
2007	320,698 (11)	11	11	12	—
2008	312,732 (10)	10	10	11	—
2009	300,201 (10)	10	10	10	—
2010	290,469 (10)	10	10	10	—
2011	288,006 (10)	9	10	9	—
2012	288,855 (9)	10	10	9	—
2013	287,425 (10)	10	10	9	—
2014	293,637 (10)	10	10	9	—
2015	285,076 (10)	9	9	9	—
Maternal Characteristics (%)
Education
$<\text{High school}$	764,090 (26)	26	31	21	$<0.0001$
High school diploma/GED	764,206 (26)	26	23	21	—
Some college	724,574 (25)	25	22	23	—
$\text{College}+$	665,993 (23)	23	24	35	—
Missingb	68,566 (2)	95	3	2	—
Age at delivery
$<20$	252,857 (8)	9	9	6	$<0.0001$
20–24	651,062 (22)	22	21	18	—
25–29	809,072 (27)	27	27	25	—
30–34	754,714 (25)	25	26	29	—
$35+$	519,700 (17)	17	17	22	—
Missingb	24 ($<1$)	92	8	0	—
Race/ethnicity
Asian/Pacific Islander	356,603 (12)	12	11	13	$<0.0001$
Black	154,047 (5)	5	6	9	—
Hispanic	1,673,517 (56)	56	59	47	—
Other	84,384 (3)	3	2	4	—
White	718,878 (24)	24	22	27	—
Kotelchuck index
Inadequate	351,729 (12)	12	13	12	$<0.0001$
Intermediate	349,946 (12)	12	12	9	—
$\text{Adequate}+$	905,545 (30)	30	29	34	—
Adequate	1,380,209 (46)	46	46	45	—
Parity					$<0.0001$
Nulliparous	1,154,875 (39)	39	40	44	—
Multiparous	1,831,556 (61)	61	60	56	—
Missingb	998 ($<1$)	93	4	3	—
Mean pre-pregnancy BMIc (SD)	2,472,066 (93)	$26\pm 6$	$26\pm 6$	$25\pm 6$	$<0.0001$
Missingb	195,033 (7)	94	4	2	—
Smoking during pregnancyc					$<0.0001$
Smoked	49,461 (2)	2	1	1	—
Did not smoke	257,7903 (97)	98	99	99	—
Missingb	39,735 (1)	92	5	3	—
TRI facility: $1+\text{within}$ $1\mathrm{km}$	48,189 (2)	2	4	3	$<0.0001$
Area-level characteristics (%)
Mean ${\mathrm{NO}}_2$ [ppb (mean ± SD)]	2,987,408 (99)	$16\pm 7$	$18\pm 7$	$19\pm 5$	$<0.0001$
Missingb	21 ($<1$)	95	0	5	—
Urban	2,651,066 (89)	89	87	97	—
Air Basin
Sacramento Valley	296,668 (10)	10	1	0.5	$<0.0001$
San Joaquin Valley	563,276 (19)	19	21	4	—
South Central Coast	178,647 (6)	6	6	1	—
South Coast	1,948,838 (65)	65	72	94	—
ICE
Quartile 1–poverty	731,431 (25)	25	31	27	$<0.0001$
Quartile 2	731,403 (25)	25	23	19	—
Quartile 3	730,283 (25)	25	19	23	—
Quartile 4–wealth	724,972 (25)	25	27	31	—
Missingb	217 ($<1$)	76	9	15	—
Oil/gas wells
Mean inactive well count (mean ± SD)	2,987,429 (100)	0	$89\pm 111$	$160\pm 191$	$<0.0001$
Mean active well count	2,987,429 (100)	0	$4\pm 4$	$32\pm 27$	$<0.0001$
Mean production volume (BOE)/d (mean ± SD)	2,987,429 (100)	0	$26\pm 26$	$599\pm 711$	$<0.0001$

Note: —, No data; BOE, barrels of oil equivalent; ICE, Index of Concentration at the Extremes.

^aANOVA or chi-square test.

^bDistribution of missingness across categories of production volume rather than percent missing in each production volume category.

^cNo covariate data available for 2006 (not included as missing), $n=\mathrm{2,667,099}$ births between 2007 and 2015.