Clinical Effectiveness of Frozen Thawed Embryo Transfer Compared to Fresh Embryo Transfer

Inclusion Criteria: * Women under 42 years of age * Presence of at least 3 embryos suitable to freeze on day 2 or 3 following fertilisation based on the centre's criteria * Written informed consent Exclusion Criteria: * Women using donor eggs/donor sperm * Women undergoing preimplantation genetic diagnosis * Women with abnormal uterine cavity shown on hysterosalpingogram or saline infusion sonogram * Women with hydrosalpinges shown on scanning and not corrected treated * Women with excessive ovarian response at risk of ovarian hyperstimulation where elective freeze is already planned * Women with serum progesterone level on day of human chorionic gonadotrophin\>1.5ng/ml or 5 nmol/L * Women whose embryos have not survived freeze-thawing in the past * Fresh transfer is planned e.g. patients with endometriosis or adenomyosis who have received prolonged downregulation * Only frozen transfer is planned e.g. patients receiving ovarian stimulation regimens which may have adverse impacts on the endometrium