How do embryos develop in the lab?

Egg retrieval – Day 0

Eggs are obtained by trans-vaginal aspiration with a needle while the patient is under anesthesia.  Fluid from each follicle is taken to the lab where the embryologists identifies the eggs.

The eggs are prepared for fertilization over a process of four hours in the lab.  If any final maturation is going to take place, this time allows for that maturation.  A mature egg is one that has gotten rid of 23 of its 46 chromosomes in preparation for fertilization when 23 chromosomes from the sperm are introduced.  Fertilization takes place by either placing many motile sperm around each egg and allowing the fertilization to take place over night in the incubator  Or intracytoplasmic sperm injection is performed where a single, motile, normally shaped sperm is injected into the egg with a tiny needle.

Day 1 after fertilization

The embryologists looks for normal fertilization the day after egg retrieval.  Each egg is checked for the presence of two pronuclei (one pro nucleus from the egg and the other from the sperm). If there are two pronuclei, then the egg was fertilized normally. The embryos that contain more or fewer than two pronuclei are chromosomally abnormal and, therefore, can never develop into a normal baby.

Days 2-5 of embryo growth

On day 3, the embryos are checked again.  Most embryos have 8 cells on day 3 of life.  They can also be graded at this point.  The better the grade, the higher the likelihood that the embryo is normal chromosomally and will continue to develop to day 5.  On day 3, each individual cell is called a blastomere.  These cells are still all the same and can become any part of the growing embryo.  By day 5, the embryos should reach the blastocyst stage.  A day 5 embryo has cells that become the placenta, called trophectoderm and cells that become the baby itself, called the inner cell mass.  Some embryos are hatching out of their shell, called a zona pellucida.  Hatching has to happen before implantation can take place.

Embryo transfer

Embryo transfers are done on day 5 most commonly.  However, if on day 3 there are few embryos that are growing well, a day 3 transfer may be performed.   You and your doctor will decide on the number of embryos to transfer.  However, Texas Fertility Center and Ovation® Fertility follow the guidelines created by the Society of Assisted Reproductive Technology (SART). These guidelines are in place to maximize your chance for pregnancy while reducing the risk of a multiple pregnancy as much as possible.  The number of embryos you and your doctor choose to transfer will be based on your age, your reproductive history, how the embryos are growing in the laboratory, and if preimplantation genetic screening has been performed.