After eggs are retrieved, they are transferred to the embryology laboratory where they are kept in conditions that support their needs and growth. The embryos are placed in small dishes or tubes containing “culture medium,” which is a special fluid developed to support the development of the embryos made to resemble that found in the fallopian tube or uterus. The dishes containing the embryos are then placed into incubators, which control the temperature and atmospheric gasses the embryos experience.A few hours after eggs are retrieved, sperms are placed in the culture medium with the eggs (regular IVF), or individual sperm is injected into each mature egg in a technique called intracytoplasmic sperm injection (ICSI).The ICSI technique attempts to achieve fertilization by the injection of a single sperm into the cytoplasm (interior) of the egg. Mature eggs are freed of surrounding cells by a combination of enzyme treatment and micro-dissection. Using special micromanipulation equipment, the eggs are individually injected with a single sperm.Injected eggs are returned to the laboratory incubator and maintained in an environment ideal for embryo growth. The eggs are returned to the incubator, where they continue to develop. Over the next few days, the dishes are inspected so the development of the embryos can be assessed.The following day after eggs have been inseminated or injected with a single sperm (ICSI), they are examined for signs that the process of fertilization is underway. At this stage, normal development is evident by the still single cell having two nuclei; this stage is called a zygote.Two days after insemination or ICSI, normal embryos have divided into about four cells. Three days after insemination or ICSI, normally developing embryos contain about eight cells. Five days after insemination or ICSI, normally developing embryos have developed to the blastocyst stage, which is typified by an embryo that now has 80 or more cells, an inner fluid-filled cavity, and a small cluster of cells called the inner cell mass.Some eggs and embryos are abnormal and so it is expected that not all eggs will fertilize and not all embryos will divide at a normal rate. The chance that a developing embryo will produce a pregnancy is related to whether its development in the lab is normal, but this correlation is not perfect. This means that not all embryos developing at the normal rate are in fact also genetically normal, and not all poorly developing embryos are genetically abnormal. Nonetheless, their visual appearance is the most common and useful guide in the selection of the best embryo(s) for transfer.In spite of reasonable precautions, any of the following may occur in the lab that would prevent the establishment of a pregnancy:
Fertilization of the egg(s) may fail to occur.
One or more eggs may be fertilized abnormally, resulting in an abnormal number of chromosomes in the embryo; these abnormal embryos will not be transferred.
The fertilized eggs may degenerate before dividing into embryos, or adequate embryonic development may fail to occur.
Preimplantation Genetic Diagnosis (PGD) and Preimplantation Genetic Screening (PGS) are two techniques that can be used during in vitro fertilization (IVF) procedures to test embryos for genetic disorders prior to their transfer to the uterus. PGD and PGS make it possible for couples or individuals with serious inherited disorders to decrease the risk of having a child who is affected by the same problem. Both of these techniques involve the use of the micromanipulator to remove a cell from an embryo. This cell is then sent to a diagnostic lab to determine the embryo’s normalcy. Acceptable embryos can then be transferred into the patient, decreasing her odds of having an affected child. PGS has also been reported to increase the pregnancy rates of some women with chromosomal disorders that result in either lower implantation rates of embryos or higher miscarriage rates.