TABLE 1.

Percentages of Singleton Preterm Births, Singleton Births With Cesarean Delivery, and Singleton Births With Induction of Labor, by Gestational Age: United States, 1991, 1996, 2001, and 2006

Gestational Age	1991 (n = 4 017 127), %	1996 (n = 3 788 111), %	2001 (n = 3 902 691), %	2006 (n = 4 129 440), %	% Change 1991–2006
Preterm births
<37 wk	9.8	9.7	10.4	11.1	13.3*
<32 wk	1.7	1.6	1.6	1.7	0.0
32–33 wk	1.3	1.2	1.2	1.3	0.0
34–36 wk	6.9	6.9	7.6	8.1	17.4*
Cesarean deliveries
≥37 wk	21.4	19.1	22.4	28.7	34.1*
<37 wk	25.1	25.6	29.9	36.9	47.0*
<32 wk	30.8	34.9	40.7	46.6	51.3*
32–33 wk	27.7	29.4	34.5	41.5	49.8*
34–36 wk	23.3	22.8	26.9	34.3	47.2*
Induction of labor
≥37 wk	10.9	17.7	21.6	24.0	120.2*
<37 wk	7.6	12.3	14.7	15.6	105.3*
<32 wk	5.4	8.1	8.9	8.9	64.8*
32–33 wk	6.9	11.2	12.8	13.4	94.2*
34–36 wk	8.2	13.5	16.2	17.3	111.0*

Note. Missing data were excluded from percentage computations. Data were derived from the natality detail files of the National Center for Health Statistics.

*P < .001.