Selective whole embryo freezing means that for some infertile women, a fresh embryo transfer may not end well and they may consider whole embryo freezing and later frozen embryo transfer. What kind of people need to undergo whole embryo freezing? 1. Patients with high risk of OHSS OHSS is a medical disease related to ovulation promotion. Mild OHSS is characterized by abdominal distension and ascites, and in severe cases, pleural fluid, aspiration difficulties, blood clots and multi-organ failure can occur, which can be life-threatening. And OHSS is important to prevent. The most effective approach is: antagonist regimen + agonist trigger to prevent early OHSS, followed by whole embryo freezing to prevent late OHSS. What are the characteristics of patients at high risk of OHSS? Young, thin, polycystic ovaries (/syndrome), allergic (immune activation), ≥35 follicles after ovulation, high estrogen levels after ovulation (>4000 pg/mL), fresh embryo transfer gestational cycle, hCG use in luteal phase, elevated vascular endothelial growth factor levels. 2. Early elevation of serum progesterone in late follicular phase Elevated serum progesterone in late follicular phase is now thought to possibly affect gene expression in the endometrium, leading to changes in the timing of the endometrial implantation window, causing changes in the endometrium to be out of sync with embryonic development, which in turn affects embryo implantation. 3. Excessive estrogen decline after egg retrieval Our center itself found by summarizing cases that in high-response patients, if the estrogen decline (compared with hCG daily estrogen) on the 2nd day after egg retrieval is more than 80%, the implantation rate of IVF fresh embryo transfer cycle will decrease. At this time, even if the serum progesterone is in the normal range or the risk of OHSS is low, it is recommended to consider whole embryo freezing and perform frozen embryo transfer. 4. Abnormal intrauterine environment The two most important factors affecting the success of IVF are embryo quality and endometrial environment. When ovulation monitoring in the ovulation promotion cycle reveals uneven endometrial echogenicity or the presence of endometrial polyps, submucosal fibroids, uterine cavity adhesions, endometritis, etc., whole embryo freezing is recommended, and frozen embryo transfer will be performed after the endometrium is treated. 5. Genetic screening For patients with advanced age (>35 years old), repeated implantation failure or repeated miscarriage, preimplantation genetic screening may be required. A small number of cells from the embryos will be taken for chromosome examination and the results will take some time to be returned, so the embryos should be frozen and preserved first. 6. Systemic conditions such as hypertension that are not suitable for transfer or pregnancy Other conditions, such as unstable control of hypertension, such as acute stage of local or systemic infection, such as the use of other drugs that are not suitable for pregnancy, etc., are recommended to be treated first and then freeze embryo transfer when the physical condition is suitable. 7. Low-response patients For low-response patients, accumulation of multiple oocytes/embryos may increase the live birth rate, but this still needs further study.