Step 3: Fertilization and embryo culture
- If sperm parameters are normal, approximately 50.000 to 100.000 motile sperm are transferred to the dish containing the eggs. This is called standard insemination.
- The ICSI technique is used to fertilized mature eggs if sperm parameters are abnormal. This procedure is performed under a high-powered microscope. The embryologist picks up a single prematozoa using a fine glass micro needle, and injects it directly into the egg cytoplasm. ICSI increases the chance the fertilization will occur if the semen sample has a low sperm count and/or motility, poor morphology, or poor progression. If there are no sperm in the ejaculate, sperm may be obtained via a surgical procedure. ICSI is always used to achieve fertilization if the sperm is surgically retrieved.
- Fertilization is assessed 16-18 hours after insemination or ICSI. The fertilized eggs are called zygotes and are cultured in a specially formulated culture medium that supports their growth. They will be assessed on the second and third day after retrieval. If sufficient numbers of embryo exhibit good growth and development, they may be selected to grow to the blastocyst stage in a specially designed culture medium. Blastocyst culture has several advantages. Embryos at this stage have a higher potential for implantation, therefore fewer embryos can be transferred on day 5 to reduce the chance of multiple pregnancies. Low numbers of embryos and poor embryo quality reduce the chances for good blastocyst development. A day 3 embryo transfer is recommended for cycles with low numbers and/or poor quality.