Factors Influencing Induced Abortion in Women Aged 18–45 Years: A Nationwide Web-Based Questionnaire Survey in Japan

Abortion remains a significant public health concern and often results from unintended pregnancies. Although abortion rates have declined, approximately 40% are still due to unplanned pregnancies. We conducted a subanalysis of the FIKA study, a survey on sexuality among Japanese women aged 18-45 years, to examine factors correlated with abortion. We analyzed responses obtained from 3228 sexually experienced women, who were categorized based on abortion history. Sources of knowledge about intercourse and contraceptives were assessed using a standardized questionnaire. A decision tree analysis was performed to identify associated risk factors. Among the participants, 410 women (12.7%) had undergone an abortion. Women who had undergone an abortion were significantly more likely to rely on friends as a source of contraceptive information than women who had not had an abortion (p = 0.0024). Participants with abortion experience showed greater variation in knowledge about contraceptive scores, with most of them being in the lowest knowledge group. Decision tree analysis revealed two key factors: sexual debut before the age of 17 years and only compulsory education. The abortion rate was 39.5% among women with both factors, compared to 7.9% among those with none. Early sexual initiation and limited education were strongly associated with abortion experience. Reliance on informal sources may also contribute to a poor understanding of reproductive health. These findings underscore the need for early, comprehensive, and formal sex education to improve sexual literacy and reduce unintended pregnancies.