Table 2.

Study population and percent preterm births by TTHM (μg/L) level averaged over different gestational periods.

TTHM (μg/L)	Study population [no. (%)]	Percent preterm (n = 2,813)
Total	37,498 (100)	7.5
First trimester
< 40	10,831 (28.9)	7.2
40 to < 60	17,588 (46.9)	7.6
≥ 60	9,079 (24.2)	7.7
Second trimester
< 40	10,604 (28.3)	7.9
40 to < 60	16,059 (42.8)	7.5
≥ 60	10,835 (28.9)	7.2
Previous 4-week average
< 40	10,300 (27.5)	8.2
40 to < 60	13,031 (34.8)	7.1
≥ 60	14,167 (37.8)	7.4
Pregnancy average
< 40	7,222 (19.3)	8.2
40 to < 60	21,399 (57.1)	7.3
≥ 60	8,877 (23.7)	7.4