Frozen Embryo Replacement (FER)is a fertility treatment by which stored embryos from a fresh IVF treatment cycle are thawed and transferred into the uterus. Frozen embryos can be replaced either in the middle of a natural menstrual cycle (Natural FER) or in a cycle where the woman takes drugs to suppress her own hormones, and then takes Hormone Replacement Therapy (HRT) to prepare her uterus for the transfer of embryos(HRT - FER).

Both are widely used methods of fertility treatment. However, to date no well designed studies have been reported in which the two methods have been compared. This study aims to compare the two treatments in order to establish the best treatment protocol.

The study design is a single centre open randomized controlled trial funded by the Oxford Fertility Unit.

100 women who are considering FER treatment at the Oxford Fertility Unit and are eligible for the study will be recruited.

After giving their written consent they will be randomised to one of 2 groups: Natural FER and HRT FER.

Patients in both groups will be asked to attend one initial visit, which is additional to patients not taking part in the study.

Subsequent visits will depend on the treatment group but will not be additional for study participants. All visits requiring ultrasound will involve recording 3-D measurements, which increases the length of each visit by approximately 10 minutes. A patient satisfaction questionnaire would be completed at the end of the cycle.

A urinary pregnancy test is performed at home 2 weeks following the embryo transfer visit. If this is positive then further visits to the unit are arranged to confirm the clinical pregnancy.

Inclusion in the study would not increase the length of time of either treatment cycle. Follow up would extend to obtaining pregnancy outcome information.

Inclusion Criteria:

• Women attending Oxford Fertility Unit planning a frozen embryo replacement cycle.
• Willing and able to give informed consent for participation in the study.
• Age at original fresh IVF cycle < 40 years old.
• At least 3 embryos frozen in storage
• First or second FER cycle
• Regular ovulatory menstrual cycles, confirmed by luteal progesterone levels >16nmol/L and with cycle length < 35 days.

Exclusion Criteria:

• Women with irregular cycles (outlined above), including amenorrhoea or oligomenorrhoea.
• Co-existing medical illness including renal, cardiac and liver disease
• Women for whom the study medication(s) are contraindicated or who have known allergic reactions to study medication(s)
• Women who have already taken part in this trial (i.e. cannot have more than one treatment cycle as a participant in this trial)