ISRCTN61225414.

Study name	Freezing of embryos in assisted conception: a randomised controlled trial evaluating the clinical and cost‐effectiveness of a policy of freezing embryos followed by thawed frozen embryo transfer, compared with a policy of fresh embryo transfer in women undergoing in‐vitro fertilization
Methods	RCT Estimated enrolment: 1086
Participants	Inclusion criteria Female partner is between 18 and 42 years of age at the start of treatment (i.e. start of ovarian stimulation) Couples who are undergoing their first cycle of IVF/ICSI treatment Both partners are resident in the UK Both partners are able to provide written informed consent Exclusion criteria Couples in whom: donor gametes are used; pre‐implantation genetic diagnosis is performed; elective freezing of all embryos is preferred or clinically indicated (e.g. severe risk of OHSS).
Interventions	Intervention arm: all good‐quality embryos will be frozen and couples will undergo frozen‐thawed ET within 3 months of the egg collection process. Couples will attend a clinic visit and additional monitoring visits before FET is performed. Standard‐care arm: women will undergo fresh ET on day 3 or 5 (after egg collection).
Outcomes	Live birth Clinical pregnancy: ultrasonic visualisation of ≥ 1 gestational sacs or definitive clinical signs of pregnancy; ectopic counts as clinical pregnancy; multiple gestational sacs count as 1 clinical pregnancy Ongoing pregnancy (pregnancy with presence of fetal heartbeat) OHSS Miscarriage rate Gestational diabetes mellitus Multiple pregnancy Hypertensive disorders of pregnancy (comprising pregnancy‐induced hypertension, pre‐eclampsia and eclampsia) Antepartum haemorrhage Preterm delivery (defined as delivery at < 37 completed weeks) Very preterm delivery (defined as delivery at < 32 completed weeks) Low birth weight (defined as weight < 2500 g at birth) Very low birth weight (defined as < 1500 g at birth) Large for gestational age (defined as birth weight > 90th centile for gestation, based on standardised charts) Small for gestational age (defined as < 10th centile for gestational age at delivery) Congenital anomaly (all congenital anomalies identified will be included) Perinatal mortality (late as well as early neonatal deaths, up to 28 days after birth)
Starting date	1 March 2015
Contact information	Christina Cole: christina.cole@npeu.oxa.c.uk
Notes	www.isrctn.com/ISRCTN61225414