Natural cycle cryo-thaw transfer may improve pregnancy outcome.

Objective: To compare natural vs. hormone replacement treatment (HRT) for cryo-thaw embryo transfer cycles in patients with frozen embryos from previous ART.

Design And Settings: Retrospective chart review of 164 patients (242 cycles) who underwent natural or HRT cryo-thaw embryo transfer between January 2002 and April 2005.

Main Outcome Measures: Pregnancy rates per transfer in natural and HRT cryo-thaw cycles.

Results: The pregnancy rate per transfer was higher with natural cycles (36.76% vs. 22.99%, P = 0.0298). There was no statistical difference in mean age, endometrial thickness, and average embryo quality in successful cycles. Mean endometrial thickness prior to transfer was greater in natural cycles (9.95 vs. 8.89 mm, P < 0.001). Mean serum estradiol levels were higher in the HRT group prior to transfer (526.1 vs. 103.8 pg/ml, P < 0.001), and were found to be lower in women who achieved pregnancy (337.1 vs. 433.3 pg/ml, P = 0.0136).

Conclusion: Hormone replacement in preparation for cryo-thaw transfer of embryos was found to be associated with decreased pregnancy rates in comparison to natural cycle cryo-thaw transfer. Greater endometrial thickness was achieved with lower serum estradiol levels in patients undergoing natural cycles, suggesting that higher estradiol levels during HRT cycle may interfere with the window of implantation.