In this cross-selectional study, the UIP prevalence among unmarried sexually active college students aged 15–26 years old was 17.7%, which is higher than the rate of 10.1 percent reported by Huang, Xiao and Wang (2020) [8], but lower than the overall level calculated by Zhou et al. (2009) [7]. The discrepancy may be partially attributable to differences in measurement [16] and differences in demographic and socioeconomic characteristics of the sampled participants [4, 16], but may also indicate a decreasing trend in UIP prevalence over time [17]. For example, the global annual UIP rate was 79 per 1000 women aged 15–49 years in 1990–1994, and the number decreased by 15 points to 64 in 2015–2019, according to the most recent figures estimated by Bearak et al. [17].

Based on the social-ecological model, multivariate Logistic regression analysis revealed that, for both males and females, UIP was significantly associated with older age (23–26 years), living with only one parent or living without parents, reporting that their family members approved premarital sex, initiating sexual activity younger than 14 years old and having a casual sex partner. Furthermore, females with multiple partners and males who came from low-income households, experienced sexual abuse, perceived difficulties in acquiring condoms and did not know how to use condoms correctly were also at higher risk of having UIP.

Undergraduates with UIP were more likely to come from broken families and report that their family members approve of premarital sex. This finding can be partly explained by two facts. One possible explanation is that those from broken families had lower perception of family strengths and therefore were less likely to feel satisfied with the quality of their communication with parents, thus contributing to the formation of anxiety, inferiority, insecurity and loneliness which were usually compensated for by various defense mechanism such as entering into a heterosexual relationships or conceiving a baby [10]. The other possible explanation is that families with more tolerant attitudes toward premarital sex exercised little supervision over their children’s dating relationships and might unwittingly push their children towards irresponsible sexual behavior [10]. Furthermore, male students from low-income households were also found to be more likely to report UIP, consistent with previous studies [2, 10, 13, 14, 18]. This finding is not surprising because those from low-income households [2, 14] often gain less knowledge about sexuality and contraception, know less about the availability of free contraceptives and have limited ability to afford modern contraceptives such as condoms, injectable hormones and oral pills, thus contributing to the occurrence of UIP.

The history of sexual abuse [11], having multiple [13, 15] and casual sexual partners [13] have been well recognized as key risk factors for UIP. Consistent with the finding of a survey conducted by Calvert et al. [15], older age and early sexual debut were also significantly associated with increased odds of UIP. This could be due to the fact that early sexual debut and older age might increase both the possibility of engaging in risky sex (i.e., discontinuation, incorrect use and inconsistent use of contraceptives) and the odds of becoming pregnant, thus leading to the occurrence of UIP.

This finding is consistent with previous research that has found no association between knowledge of sexual health and UIP [15]. This phenomenon can be explained by at least two reasons. First, a one-item question to measure respondents' knowledge about pregnancy prevention methods focused only on condom-use and not on any other contraceptive method [15]. Second, condom-use knowledge might exert indirect effects [15] through skills or self-efficacy captured in this study such as condom-use skills and perceived difficulties in acquiring condoms.

Several limitations of this study should be taken into consideration. First, because this study was cross-sectional in nature, the cause-effect relationship between the probability of UIP and a range of familial, demographic and individual factors cannot be established. Second, nonrandom sampling procedure in the present study might produce biased parameter estimates due to the lack of representativeness of the sample and thus limit the generalizability or external validity of the results. Third, data on sexuality and UIP were obtained by self-report and might be subject to recall and social desirability bias. Fourth, due to lack of couple-level data [18, 19], the agreement between the females' self- and partner-reports of experiencing UIP was not assessed in this study. Finally, other potential factors which have not been studied extensively include unhealthy behaviours such as smoking, drinking and drug abuse [16], partners and peers [1], community characteristics (e.g., community media exposure, community fertility norm and community education) [3], as well as policy or relevant legislation [1].

Our findings have several important implications. First, target older students and those engaging in risky sexual behaviors. Older age, early sexual debut and sexual abuse were found to be significantly associated with an increased probability of engaging in risky sexual behaviors and experiencing UIP. Furthermore, correct and consistent condom use can prevent both UIP and HIV/STD infections. Therefore, the 100% Condom Use Program should immediately be promoted to target students with these characteristics.

Second, work with male students to improve condom-use skills and improve the availability of free condoms. Males who came from low-income households, perceived difficulties in acquiring condoms and did not know how to use condoms correctly were found to have a higher proportion of UIP. In order to promote condom use, the first and foremost intervention is to work with male students to improve their condom-use skills [20]. Also, intervention should focus on identifying barriers to condom acquisition and delivering free condoms to male students (especially those economically disadvantaged).

Third, optimize the involvement of parents and other family members in their children’s sex education. Our study showed that approximately 40 percent of the students came from broken families, and adverse family events such as parental absence, parental separation or divorce might result in inadequate care and support and potentially contributed to the occurrence of UIP. Furthermore, individuals who report that their family members approve of premarital sex were more likely to experience UIP. Therefore, students’ family members, especially their parents must be involved in educational programs to foster their values related to responsible sexual behavior and wise decision making [21,22,23,24]. An optimal family centered approach is expected to create an environment where parents communicate with their adolescent children about sexual issues more frequently and with greater ease [21,22,23,24].