Table 3. The correlation between the mean prevalence and potential sources.

Covariate	Age (year)	GA (week)	Survey year	AP	PCS	MP	QS
r	−0.294	−0.301	−0.400	0.281	0.216	0.534	0.108
P	0.154	0.1975	0.031	0.330	0.334	0.031	0.578

AP: placenta on anterior wall; GA: gestational age; MP: multiperous; PCS: previous cesarean section; QS: quality score.