First-Trimester Uterine Artery Doppler Indices and Pregnancy Outcomes in Naturally Conceived and Frozen-Thawed Embryo Transfer Cycles.

: The role of luteal phase support (LPS) in frozen-thawed embryo transfer (FET) cycles has garnered increasing interest, particularly regarding its influence on uterine perfusion and pregnancy outcomes. This study aimed to investigate the effect of different oral LPS regimens on first-trimester uterine artery Doppler indices and their association with early pregnancy outcomes in naturally conceived and FET pregnancies. : This retrospective cohort study included 289 singleton pregnancies comprising spontaneous conceptions, FET cycles supported with oral micronised progesterone, and FET cycles supported with oral dydrogesterone. The uterine artery pulsatility index (PI) was measured via Doppler ultrasound during the first trimester. Group comparisons were performed using non-parametric tests. Multivariable regression analyses were used to assess independent predictors of PI and associations with gestational diabetes and low birth weight. : Uterine artery PI values differed significantly among the groups ( < 0.001). The lowest PI was observed in the dydrogesterone group, followed by the naturally conceived and micronised progesterone groups. A higher maternal body mass index (BMI) was independently associated with lower PI ( = 0.009), while maternal age showed no significant effect. No significant associations were found between PI or maternal characteristics and adverse outcomes such as gestational diabetes or low birth weight. : Dydrogesterone was associated with more favorable uterine artery Doppler indices in early pregnancy, suggesting improved uteroplacental adaptation in FET cycles. These findings support further prospective research to determine the clinical impact of LPS regimens on placental development and perinatal health.