Table 1. The basic characteristics of the study population (2007–2010).

	2007	2008	2009	2010	p value for trends
Age (years)	30.04 ± 3.93	30.26 ± 3.97	30.44 ± 3.98	30.73 ± 3.99	<0.001
Older maternal age (≥35 years)(%)	12.56	13.96	15.03	16.66	<0.001
Multiparity (%)	46.52	46.56	46.77	48.37	<0.001
Multiple pregnancies (%)	1.43	1.41	1.40	1.38	<0.001

Data are mean±standard deviation or %.