Interpregnancy Interval After Clinical Pregnancy Loss and Outcomes of the Next Frozen Embryo Transfer

IMPORTANCE: The optimal interpregnancy interval (IPI) after a clinical pregnancy loss (CPL) remains controversial. Few studies have addressed the role of the IPI after a preceding CPL during in vitro fertilization (IVF) treatment. OBJECTIVE: To evaluate the association between different IPI lengths after a preceding CPL and pregnancy outcomes of the next frozen embryo transfer (FET). DESIGN, SETTING, AND PARTICIPANTS: This retrospective cohort study was conducted using data from the Center for Reproductive Medicine of Shandong University in China. The study included women who underwent frozen-thawed blastocyst transfer between July 1, 2017, and June 30, 2022, within 1 year after a preceding CPL during IVF treatment. Follow-up for pregnancy outcomes was completed for all participants on March 31, 2023. Data analysis was performed from April to May 2023. EXPOSURES: Interpregnancy interval length was classified as less than 3 months, 3 to less than 6 months, or 6 to 12 months. MAIN OUTCOMES AND MEASURES: Outcomes included live birth, conception, clinical pregnancy, pregnancy loss, preterm birth, small or large for gestational age, and low birth weight. Multivariable logistic regression analysis was conducted to evaluate the association between IPI and pregnancy outcomes by adjusted odds ratios (AORs). RESULTS: This study included 2433 women (mean [SD] age, 31.8 [4.6] years) who received IVF treatment. There were 338 women (13.9%) with an IPI of less than 3 months, 1347 (55.4%) with an IPI of 3 to less than 6 months, and 748 (30.7%) with an IPI of 6 to 12 months. The median (IQR) IPI lengths for the 3 groups were 77 (65-85), 128 (109-152), and 234 (202-288) days, respectively. Compared with an IPI of 6 to 12 months, shorter IPIs (<3 and 3 to <6 months) were associated with decreased odds of clinical pregnancy (AOR, 0.70 [95% CI, 0.53-0.92] and 0.79 [0.65-0.95]), live birth (AOR, 0.64 [95% CI, 0.48-0.85] and 0.74 [0.61-0.90]), and healthy live birth (AOR, 0.63 [95% CI, 0.46-0.87] and 0.79 [0.64-0.98]). Compared with women with an IPI of 6 to 12 months, women with shorter IPIs (<3 and 3 to <6 months) had a higher risk of total pregnancy loss (AOR, 1.87 [95% CI, 1.31-2.67] and 1.29 [1.00-1.66], respectively). CONCLUSIONS AND RELEVANCE: The results of this study suggest that delaying the next FET for at least 6 months after a preceding CPL was associated with beneficial pregnancy outcomes, considering that a decreased likelihood of achieving clinical pregnancy and live birth was observed among women with shorter IPIs. Further prospective studies are needed to confirm these findings.