Background <p>In hormone replacement therapy-frozen-thawed embryo transfer (HRT-FET) cycles, endogenous LH levels may still rise, and the relationship between this and pregnancy outcomes is unclear. The purpose of this study was to investigate the correlation between the serum LH levels before progesterone administration in HRT-FET cycles and the live birth rate (LBR).</p> Methods <p>A total of 13 720 HRT-FET cycles were divided into four groups based on serum LH levels according to the quartiles. Meanwhile, subgroup analyses were performed based on the use of pituitary down-regulation to evaluate the independent effects of serum LH levels on pregnancy outcomes. We used multivariate logistic regression analysis to adjust for potential confounding factors.</p> Results <p>In the overall, the 51-75th percentile group showed significant differences in LBR and miscarriage rate compared to the reference group (<i>P</i> = 0.010; <i>P</i> = 0.004), and the &gt; 75th percentile group showed significant difference in biochemical pregnancy rate compared to the reference group (<i>P</i> = 0.022). In the non-pituitary down-regulation group, the 51-75th percentile group and the reference group exhibited significant differences in LBR and miscarriage rate (<i>P</i> = 0.004), and the 26–50th percentile group showed significant difference in miscarriage rate compared to the reference group (<i>P</i> = 0.026). In the pituitary down-regulation group, the &gt; 75th percentile group showed significant difference in biochemical pregnancy rate compared to the reference group (<i>P</i> = 0.045).</p> Conclusion <p>In HRT-FET cycles, low serum LH levels prior to progesterone administration may be associated with poor pregnancy outcomes. For patients presenting with low LH levels, we may recommend deferring the FET cycle to reduce poor pregnancy outcomes.</p>