Assessment of oocyte and embryo quality

Regarding clinical outcomes, the development conditions of oocytes and embryos were observed and recorded by professional staff. MII oocytes with their corresponding polar bodies and fertilized oocytes with pronuclei were observed at 4–6 h and 16–18 h after insemination, respectively. Good quality embryos and transferable embryos were judged according to a cleavage embryo scoring system [41] at day 3. Professional staff observed the number of cleavage spheres, their symmetry, cytoplasmic morphology, and the number of fragments produced during division. According to these parameters, the embryos were divided into four grades. Grade I–II embryos have equally sized blastomeres with 0%–20% fragmentation. Grade III embryos have unequal sizes and 20% fragmentation. Grade IV embryos have low developmental potential with an abnormal appearance. The grade I–II embryos were recorded as good quality embryos, and grade I–III embryos were recorded as transferable embryos. Only transferable embryos were frozen on day 3, after the assessment of embryo quality. Concurrently, grade IV embryos were discarded. When the patient had reached the condition of embryo transfer, two embryos were thawed and transferred, including endometrial thickness between 8 and 12 mm, E2 > 100 IU/L, and LH <10 IU/L. High-level embryos were transferred as a priority. For implantation, we included only the first transfer from each patient, and all embryos transferred were vitrified-warmed.

The fertilization rate of IVF = the number of zygotes / the total number of retrieved oocytes × 100%.

The fertilization rate of ICSI = the number of zygotes / the number of MII stage oocytes × 100%.

The transplantable embryo rate = the number of transferable embryos (grade I–III) / the number of total embryos × 100%.

The good quality embryo rate = the number of good quality embryos (grade I–II) / the number of total embryos × 100%.