The impact of prior hormonal intrauterine device (IUD) use on endometrial lining thickness in the fertility clinic setting: a retrospective cohort study.

Purpose: Despite wide-spread use of intrauterine devices (IUDs), research on an IUD's long-term impact on endometrial growth and function is limited. Endometrial atrophy is a known mechanistic action of hormonal IUDs, and characterizing its reversibility is critical. Our study compared endometrial lining thickness (EMT) in patients undergoing fertility treatment with and without prior hormonal IUD use.

Methods: This was a retrospective cohort study of 163 patients aged 18-45, each with at least 1 treatment cycle with ultrasound monitoring at a single academic institution between 2018 and 2023. Patients with prior hormonal IUD use (n = 72) were compared with patients without prior IUD exposure (n = 91). Primary outcomes included mid-cycle endometrial thickness (EMT) in the first monitored cycle, thin lining (< 7 mm), and persistently thin lining (< 7 mm in ≥ 2 cycles). Secondary outcomes included trilaminar endometrial pattern. Multivariable regression models adjusted for age, BMI, parity, infertility diagnosis, uterine pathology, history of uterine procedures, miscarriage history, and treatment protocol type. Exploratory analyses evaluated duration of hormonal IUD exposure.

Results: Median EMT in the first monitored cycle was significantly lower among patients with prior hormonal IUD exposure compared with patients without prior IUD exposure (6.70 mm vs 8.60 mm, p < 0.001). Thin endometrium in the first cycle occurred in 63% of HORMONAL IUD users versus 16% of controls (RR 3.79, 95% CI 2.31-6.23, p < 0.001). Among patients with multiple cycles, persistently thin lining occurred in 64% of hormonal IUD users versus 10% of controls (RR 6.32, 95% CI 3.05-13.13, p < 0.001). In multivariable analyses, prior hormonal IUD exposure remained associated with thinner EMT in the first cycle (adjusted mean difference - 1.9 mm, 95% CI - 2.6 to - 1.2, p < 0.01) and increased risk of thin lining (adjusted RR 4.09, p < 0.01). Cycle-level analyses similarly demonstrated thinner endometrium and higher rates of thin lining among hormonal IUD users after adjustment for treatment protocol type. Longer duration of hormonal IUD use was not significantly associated with greater risk of thin lining.

Conclusions: Prior hormonal IUD use was significantly associated with thinner EMT in patients undergoing fertility treatment. While thinner lining after IUD use is concerning in this specific population, future prospective studies are needed to evaluate the clinically relevant outcomes of live birth.