Fig. 2

Study protocol. IVF stimulation protocols, trigger agents (hCG) and fertilization (IVF or intracytoplasmic sperm injection) will follow standard clinical practice. Patients will be randomized to p.f.d. 3 or p.f.d. 5 ET on day 1 of embryo development if they have ≤5 embryos available. Selection of embryos will be based on morphology and the number of embryos transferred will be based on the current American Society of Reproductive Medicine (ASRM) guidelines. Following the embryo transfer, clinical care will follow standard practice. Participants will receive vaginal progesterone for luteal support starting the day after egg retrieval until 8 weeks gestation or until 10 days after fresh embryo transfer, if the serum pregnancy test is negative. Any unused embryos will be cultured to p.f.d. 5–7 and cryopreserved by vitrification per standard clinical protocols. A pregnancy test (serum hCG) will be performed on day 10 following embryo transfer. If negative, any cryopreserved embryos will be transferred in subsequent frozen embryo transfer cycles as per standard protocol. We will administer intra-muscular progesterone with or without vaginal progesterone for luteal support in thaw cycles after the endometrial lining reaches a minimal trilaminar endometrial thickness of 7 mm (=day 1) with embryo transfer in the afternoon of day 6