3.1 The characteristics of the study population
A total of 6640 pregnant women were recruited, we excluded 139multiplets. The final sample size for the study was 6501 women with singleton live births, included 285 PTBs and 6216 full-term births, the preterm rate was 4.38%.
Compared to women with full-term birth, mothers who experienced PTB tended to be older, be housewife, have more than one parity, be cesarean, have pregnancy complications, and lower family income (Table 1).