Pre-pregnancy intrauterine device use is associated with a reduced risk of subsequent preterm birth: a large population-based cohort study

Abstract Background The effect of pre-pregnancy intrauterine device (IUD) use on subsequent preterm birth (PTB) remains unclear. To investigate the association between IUD use before pregnancy and subsequent PTB. Methods A total of 242,009 women who participated in the National Free Preconception Health Examination Project (NFPHEP) in Yunnan from 2013 to 2019 were included in the study. All study participants were classified into three groups according to their use of pre-pregnancy contraceptive methods: non-method users, IUD users, and other method users. We used multivariable Poisson regression model to investigate the association between the use of an IUD before pregnancy and subsequent PTB. Further models analyzed the multiplicative and additive interactions between pre-pregnancy IUD use and county deprivation. Results Of all the participants, 45,772 (18.9%) used IUDs before pregnancy, 39,627 (16.4%) used other contraceptive methods, and 156,506 (64.7%) were non-method users. The overall PTB rate was 4.8% (95% confidence interval 95% CI, 4.7–4.9%), and women in the IUD group had a significantly lower PTB rate (4.3%, 95% CI 4.1–4.5%) than women in the non-method users (4.9%, 95% CI 4.8–5.1%) and other method groups (4.7%, 95% CI 4.5–4.9%). IUD use before pregnancy was associated with a reduced risk of subsequent PTB (model 1: adjusted relative risk aRR 0.84, 95% CI 0.79 to 0.88; model 2: aRR 0.84, 95% CI 0.79 to 0.90). In counties with a normal level of development, IUD users had a 30% lower risk of subsequent PTB than non-users (aRR 0.70, 95% CI 0.63 to 0.77). Compared with non-IUD users from the least developed counties, those from counties with a normal level of socioeconomic development had the lowest risk of subsequent PTB (aRR 0.72, 0.62 to 0.83). The additive interaction between pre-pregnancy IUD use and low level of county development was statistically significant (relative excess risk due to interaction RERI -0.27, -0.40 to -0.13). Conclusion Pre-pregnancy IUD use is associated with a reduced risk of subsequent PTB. Pre-pregnancy IUD users in counties with a normal level of development were associated with a lower risk of subsequent PTB than their counterparts in the least developed countries.