Once the eggs are put with the sperm, the petri dishes are placed into an incubator. Fertilization will take a minimum of 12 hours to occur so most clinics will re-evaluate the eggs the next morning for fertilization. It is expected that some eggs will fertilize normally, called a 2PN stage, some will not fertilize and some will fertilize abnormally (1PN or 3PN). Because of this, there will be another reduction in the number of eggs available to use. With fertilization, the egg is now referred to as an embryo. A typical threshold to evaluate fertilization is to expect a minimum of 50% of the available (mature) eggs to fertilize. If that threshold is not reached, it could be because of egg quality, sperm quality or lab quality. Only the fertilized embryos can be cultured to develop into embryos that can be transferred back into the womb, so this number essentially establishes how many might be available for transfer. Fertilized embryos are now transferred into a new culture media and placed in incubators that have a specific gas content and temperature to allow them to grow.
We will continue this discussion soon with the next installment, "Step Six: Embryo Development". Thank you for joining me today!
Edward J. Ramirez, M.D. F.A.C.O.G.
Medical Director, Monterey Bay IVF