Table 2. Change in numbers and rates of preterm birth in association with each obstetric outcome with time.

		Number					Rate per 1000 Singleton Births
Obstetric Outcome	Gestation (wk)	1980–1984	1985–1989	1990–1994	1995–1999	2000–2004	1980–1984	1985–1989	1990–1994	1995–1999	2000–2004	% Increase or Decrease	p (Chi-Square Test for Linear Trend)
All singleton births	All	324,725	319,361	313,659	280,613	251,716
Singleton preterm birth	24–36	17,659	17,822	18,216	17,070	16,094	54.38	55.81	58.08	60.83	63.94	17.6%	<0.01
	24–27	597	654	988	947	973	1.84	2.05	3.15	3.37	3.87	110.3%	<0.01
	28–31	2,291	2,228	2,274	2,017	1,910	7.06	6.98	7.25	7.19	7.59	7.6%	<0.01
	32–36	14,771	14,940	14,954	14,106	13,211	45.49	46.78	47.68	50.27	52.48	0.15%	<0.01
Spontaneous preterm births with pPROM	24–36	1,093	1,260	2,512	2,727	2,450	3.37	3.95	8.01	9.72	9.73	189.2%	<0.01
	24–27	63	91	175	159	144	0.19	0.28	0.56	0.57	0.57	194.9%	<0.01
	28–31	196	221	375	342	308	0.60	0.69	1.20	1.22	1.22	102.7%	<0.01
	32–36	834	948	1,962	2,226	1,998	2.57	2.97	6.26	7.93	7.94	2.09%	<0.01
Spontaneous preterm births without pPROM	24–36	13,160	12,803	11,673	9,973	9,897	40.53	40.09	37.22	35.54	39.32	−3.0%	<0.01
	24–27	496	509	648	495	594	1.53	1.59	2.07	1.76	2.36	54.5%	<0.01
	28–31	1,589	1,466	1,442	1,248	1,295	4.89	4.59	4.60	4.45	5.14	5.1%	<0.01
	32–36	11,075	10,828	9,583	8,230	8,008	34.11	33.91	30.55	29.33	31.81	−0.07%	<0.01
Induction/elective caesarean delivery	24–36	3,394	3,743	4,005	4,346	3,722	10.45	11.72	12.77	15.49	14.79	41.5%	<0.01
	24–27	37	53	162	293	235	0.11	0.17	0.52	1.04	0.93	719.4%	<0.01
	28–31	505	538	452	427	305	1.56	1.68	1.44	1.52	1.21	−22.1%	<0.01
	32–36	2,852	3,152	3,391	3,626	3,182	8.78	9.87	10.81	12.92	12.64	0.44%	<0.01

pPROM, premature rupture of membranes.