Impact of interpregnancy interval after pregnancy loss on clinical pregnancy and neonatal outcomes of subsequent frozen-thawed embryo transfer cycles: a retrospective cohort study

The optimal interpregnancy interval (IPI) before reinitiating frozen-thawed embryo transfer (FET) after pregnancy loss remains uncertain, particularly in women undergoing assisted reproductive technology. Current guidelines recommending a waiting period are largely based on spontaneous conception data and may not apply to FET cycles. This study aimed to evaluate whether the length of IPI following biochemical or clinical pregnancy loss affects subsequent reproductive and neonatal outcomes in an infertile FET population. In this retrospective cohort study, 2,620 infertile women who experienced a biochemical or clinical pregnancy loss after a preceding FET cycle and subsequently underwent a consecutive FET cycle derived from the same oocyte retrieval at a tertiary academic center between January 2011 and December 2022 were included. IPI was defined as the interval from the end of the failed pregnancy to the subsequent embryo transfer and categorized as < 6 months, 6–12 months, or 12–24 months. Live birth was the primary outcome. Secondary outcomes included conception, clinical pregnancy, pregnancy loss, and adverse neonatal outcomes. Multivariable logistic regression models were used to estimate adjusted odds ratios (aORs) with 95% confidence intervals (CIs). Using the IPI of 6–12 months as the reference, a shorter IPI (< 6 months) was not associated with a reduced likelihood of clinical pregnancy among women with prior biochemical pregnancy loss (aOR 1.29; 95% CI 0.88–1.89) or clinical pregnancy loss (aOR 0.93; 95% CI 0.71–1.21). Similarly, the odds of live birth were comparable between women who reinitiated FET within 6 months and those who waited 6–12 months, regardless of the type of preceding pregnancy loss (biochemical pregnancy loss: aOR 1.18; 95% CI 0.80–1.74; clinical pregnancy loss: aOR 0.94; 95% CI 0.73–1.23). Extending the IPI to 12–24 months did not confer additional benefits for reproductive outcomes. Rates of adverse neonatal outcomes, including preterm birth, low birth weight, and small for gestational age, were similar across all IPI categories. Among infertile women undergoing FET after a biochemical or clinical pregnancy loss, reinitiating FET within 6 months was not associated with compromised reproductive or neonatal outcomes. These findings support the safety of early FET reinitiation and suggest that delaying subsequent treatment in the absence of medical indications may be unnecessary.