In Vitro Fertilization
What Happens in the Lab
The IVF laboratory is a critical part of the IVF process. Eggs are placed into dishes where they will be counted and evaluated.
Approximately 6 hours after egg retrieval, fertilization is accomplished by placing a small concentration of sperm onto each egg, or by ICSI (where a single sperm is injected into an egg.)
Approximately 18 hours after retrieval (the day after egg retrieval) we will evaluate fertilization of the eggs. Normally, fertilized eggs will have two pronuclei.
Zygotes are fertilized eggs. It has two pronuclei. One pronucleus is a packet containing half of the mother’s DNA. The other pronucleus contains half of the father’s DNA. This genetic material will unite to form the full complement of genetic material that will make an embryo.
The embryos are cultured (or grown) in incubators with an environment designed to mimic the human fallopian tube. A careful balance of carbon dioxide, oxygen, and nutrients are maintained at the correct pH (acid content) to promote embryo growth.
Only normally fertilized eggs are kept in culture. Not all embryos will grow in culture. Some of them will be genetically abnormal. Others cease to grow for unknown reasons. Embryos destined to result in pregnancy will progress through several well defined stages including zygote, cleavage and blastocyst stages.
Cleavage stage embryos are those which are dividing into progressively smaller cells. Two days after fertilization, normal embryos are between 2 and 6 cells. Three days after fertilization, most normal embryos are between 7 and 9 cells.
Four days after fertilization, most normal embryos have reached the morula stage (too many cells to reliably count with a microscope).
Blastocysts are those embryos which are nearly ready to implant. Blastocysts have developed three distinct regions: 1) an inner cell mass, which becomes the baby, 2) the trophoectoderm, which becomes the placenta, 3) and the fluid filled cavity, which will form a structure called the yolk sac.
Grade 1-2 embryos have a 20-35% implantation rate, depending on the age of the woman.
No evaluation of the embryos occurs at the morula stage (four days after retrieval) because the cell mass rarely has distinct features which are easy to grade.
On the fifth day after fertilization, most normal embryos have reached the blastocyst stage, as mentioned above. We grade blastocysts on a different scoring system than day 2 or 3 embryos. Each blastocyst will be given a letter grade, A, B, C, D, E, or F, for the relative quality for each of the three regions: the inner cell mass, the trophoectoderm and the cavity. A top quality embryo will be graded AAA, though few embryos receive this high a grade.
Embryos BBC and higher have an excellent chance of resulting in pregnancy. Top quality blastocysts have a 45-65% chance of resulting in pregnancy, depending on the age of the woman.
The day of your embryo transfer will be determined by your doctor in conjunction with the embryologist.
The very best embryos will be selected for embryo transfer. The number of embryos transferred will depend on several factors, including your preference, the quality of the embryos and the ASRM Embryo Transfer Guidelines.
If you have other embryos which are suitable for freezing, they can be cryopreserved (For details, see Embryo Freezing).
It is critical to understand that morphology scoring of the embryos as described above does NOT evaluate the genetic potential of the embryo (ie, will the baby have a normal number of chromosomes) or the potential health of a baby that grows from the embryo. While preimplantation genetic testing can investigate the chromosome number of the embryo (see PGD), no test can guarantee a healthy baby after IVF.