Associated factors for TTP
Cox regression calculations revealed factors that considerably impaired the probability of pregnancy (Table 3). In an adjusted model, older groups had significantly longer estimated TTP than the group aged 20–24 years. Women with lower educational attainment tended to have longer TTPs than those with higher educational attainment (adjusted FOR=1.31, 95% CI: 1.04–1.64 for women with college and higher educational level). Women without full-time work were more likely to become pregnant than employed women (adjusted FOR=1.60, 95% CI: 1.48–1.73 for unemployed women). As shown in Table 1, couples with a higher annual household income had impaired probability of conception than couples with a lower income (adjusted FOR=0.75, 95% CI: 0.67–0.84 for households with an income of more than 20,000 RMB). With respect to reproductive characteristics, couples who had a longer duration of cohabitation (FOR=1.06, 95% CI: 1.05–1.07) or were multiparous (FOR=1.13, 95% CI: 1.02–1.25) had shorter TTP than those who had a shorter duration of cohabitation or were uniparous. Other factors that could potentially reduce the probability of pregnancy included toxic exposure in men, longer menstrual cycle interval, and history of abnormal pregnancy. All other factors evaluated in our study did not have a statistically significant influence on pregnancy rates.