The fertility treatment in vitro fertilisation (IVF), sometimes including intra-cytoplasmic sperm injection (ICSI), involves the creation of embryos in a laboratory. These embryos are then transferred into the womb of the patient with the hope of a resulting pregnancy and live birth. Embryos can also be cryopreserved (frozen) and stored, and then later replaced in a cycle called frozen embryo transfer (FET). There are several methods of preparing the patient's womb to receive the frozen-thawed embryo(s) but commonly embryos are replaced during a medicated cycle. Usually oestrogen and progesterone are administered to prepare the womb lining for embryo transfer at the appropriate time, and in addition a drug called a GnRH antagonist is administered to prevent a women's own hormones from interfering with this process as it is thought this might lead to higher numbers of cycles being cancelled. However, there is some suspicion that this drug (GnRH antagonist) may not be required and that women are using this drug unnecessarily.

Some clinics do not use GnRH antagonists in FET cycles, but the investigators do not know if they have higher rates of cancelled cycles as a result.

This pilot study aims to compare cycles of medicated FET using oestrogen and progesterone, either with or without pituitary suppression in the form of GnRH antagonist (Cetrotide), in patients over the next 18 months who are planning FET cycles at Oxford Fertility, UK to find out if both give the same chance of having a baby, which treatment is better for patients and to assess the feasibility of undertaking a future larger study. Cetrotide is a marketed and well-known medication and any risk or serious adverse effects are unlikely. The study is an open label prospective randomised controlled trial. Funding for the medication (Cetrotide) is provided by Oxford Fertility.

Inclusion Criteria:

• Willing and able to give informed consent for participation in the study.
• Planning to undergo medicated FET
• Blastocyst embryo in storage available for transfer

Exclusion Criteria:

• Previously randomised into the trial.
• A history of 3 or more consecutive implantation failures (after fresh or frozen embryo transfer).
• A history of recurrent miscarriage (3 or more consecutive miscarriages).
• Contraindication to the use of medications for FET cycle.
• Biopsied embryos.
• Donor embryos or eggs (use of donor sperm is not excluded).