Embryo Stages

The day that eggs are retrieved and combined with sperm is considered DAY 0 of embryo development. Couples are told how many eggs were retrieved before they leave the Fertility Center. Semen can be acquired that day or prepared from a thawed sample that was cryogenically preserved, and two methods can be used for fertilization. Conventional or standard fertilization involves adding sperm to each dish containing a mature egg and waiting to see if fertilization occurs. Intracytoplasmic sperm injection (ICSI) is another approach that injects one sperm into each mature egg to give the fertilization process a head start. Retrieved eggs that are not mature will not fertilize.

DAY 1 – Embryologists check each dish to see if fertilization has occurred to create zygotes. About 24 hours after insemination, the zygotes typically begin dividing. This early stage of division or “cleavage” gives an indication of the strongest embryos. Each embryo is cultured separately, unfertilized eggs are discarded, and the patient receives a voicemail update.

Day 2 – No one even looks at the embryos on this “rest” day, which keeps them warm and undisturbed.   

Day 3 – In addition to checking for cleavage on the third day, 
embryologists begin to count the number of cells in each embryo. 
The ones with three cells or less are all transferred into one dish and observed for two more days. Embryos with four or five cells are a little slow in their development but can catch up; six- to ten-celled embryos are considered “normal” while those with eight cells are ideal.  By this point in development, the quality of each embryo becomes more defined, and any fragmentation can be observed. The embryo stops living off maternal reserves and must generate energy on its own. Transfers rarely happen on this day because the true potential of the embryos is yet to be seen. A member of the Fertility Center staff calls the patient to share a second update on the embryos.

Day 4 – Another day of rest lets nature take its course.

Day 5 – By now, each embryo is typically 60-120 cells and called a blastocyst. These are graded 
based on quality, and the top one or two typically are transferred. Occasionally, any unused 
embryos are cryogenically frozen at this point, but most continue to grow overnight.

Day 6 – The majority of extra embryos are frozen. A very small percentage of transfers do take place on Day 6, especially if the embryos appeared to be growing a little slower than normal. Embryos that stopped developing and poor quality embryos that won’t survive the cryogenic process are discarded at this time since they are not viable for pregnancies.