Objective: To evaluate five luteal support protocols in women with low serum progesterone (<10 ng/mL) undergoing HRT-FET. Methods: Randomized controlled trial at two centers including 200 women under 35 with unexplained infertility. Groups: (1) 600 mg vaginal, (2) 800 mg vaginal, (3) 600 mg vaginal + 50 mg IM, (4) 600 mg vaginal + 25 mg SC, (5) 600 mg vaginal + 30 mg oral. Results: Groups 3 and 4 achieved significantly higher serum progesterone (p < 0.001), higher clinical pregnancy (70%, 68%), and higher live birth (84%, 83%) compared to Groups 1, 2, and 5. Early pregnancy loss was lower in Groups 3 and 4. Conclusions: Combined vaginal and injectable progesterone improved outcomes compared to monotherapy.