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OBJECTIVE: To determine whether there is any difference between the outcomes of two standard treatment protocols for frozen embryo replacement (FER): natural and down-regulated hormone replacement treatment (HRT). DESIGN: Open, single-center, randomized, controlled pilot trial. SETTING: Private fertility clinic. PATIENT(S): Women (n = 159) planning an FER cycle at the Oxford Fertility Unit, aged <40 years at the time their embryos were frozen; with at least one blastocyst or two cleavage-stage embryos in storage; regular ovulatory cycles; and at most two previous FER cycles. INTERVENTION(S): Eligible participants were recruited and randomized between March 2010 and July 2012 into one of two standard FER treatment groups: natural (n = 80) menstrual (Natural) or GnRH agonist/HRT (n = 79) cycles. MAIN OUTCOME MEASURE(S): Live birth rate after replacement of frozen-thawed embryos, clinical pregnancy rate, implantation rate, and cycle cancellations. RESULT(S): A total of 159 women were randomized (80 Natural; 79 HRT), and 145 had ET and completed the study (72 Natural; 73 HRT). Pregnancy outcomes were not significantly different between the two groups. The live birth rates were 26.3% (Natural) and 31.7% (HRT) per randomized patient. Per ET/protocol the live birth rates were 29.2% and 34.2%. The implantation rates were 24.3% and 26.0%, and there were three twin births in the Natural and five in the HRT arms. CONCLUSION(S): The findings of this pilot study support the suggestion that for women with ovulatory cycles undergoing FER, the outcomes are similar between natural and HRT protocols. CLINICAL TRIAL REGISTRATION NUMBER: NCT00843570. Registered at

Original publication




Journal article


Fertil Steril

Publication Date





915 - 920.e1


IVF, In vitro fertilization, frozen embryo replacement, Adult, Embryo Implantation, Embryo Transfer, Female, Fertilization in Vitro, Freezing, Hormone Replacement Therapy, Humans, Menstrual Cycle, Oocyte Donation, Pilot Projects, Pregnancy, Pregnancy Outcome, Pregnancy Rate