A Guide to Embryo Development After Retrieval

I just had my egg retrieval, what happens next?? What is going on behind that lab door??? These are questions patients ask all of the time.  Our embryology staff created this helpful guide to embryo development from Retrieval Day to Day 6.

Day 0 – Retrieval day is what the lab refers to as day 0. The ASC nurse will provide you with a final egg count before you leave the recovery area. 4-6 hours following the time of the egg retrieval the embryologist will initiate the fertilization process of the eggs. Fertilization is attempted using one of two methods, standard insemination or ICSI. Standard insemination involves placing a required number of washed, motile sperm into the culture drop containing an egg. ICSI is a more involved process where the embryologist uses a specialized needle and microscope to catch a single sperm to inject directly into the cytoplasm of the mature egg. Once insemination or ICSI occurs the eggs are placed back into the incubator to allow time for fertilization to occur.

Standard Insemination                                        ICSI


standard insem






Occasionally, the method of insemination may change from the original plan based upon the results of the semen analysis of the sperm collected on the day of the retrieval. The lab will perform a thorough sperm analysis on the retrieval day and will make recommendations to the physician as to which insemination method may work best for fertilization based upon the sperm results that day.

Day 1Day 1 is what the lab refers to as fertilization check day. Approximately 16-20 hours after the insemination process takes place the embryologist will check to see if fertilization has occurred. The embryologist is looking for evidence of 2 nuclei (pn) – one from the egg and one from the sperm. This is how the embryologist determines if normal fertilization has occurred. Any more or any less nuclei present and the embryo is considered abnormally fertilized and is not kept in culture. Once normally fertilized, the embryos are placed back into the incubator and are not disturbed until day 3. The embryos are grown in a specially designed culture dish which contains a small drop of culture medium overlaid with oil. This embryo culture medium contains the proteins, amino acids, and enzymes that mimic the fluid found in the fallopian tubes essential for embryo development. One factor (of many) that contributes to successful embryo development is the constant maintenance of temperature and pH level of this embryo culture medium. The temperature and culture medium pH level is dependent upon the CO2 environment and the heat that is provided by the incubators. Since the embryos are grown in a very small drop of culture medium it does not take long for temperature to drop and the pH level to change in that drop containing the embryo once the dish is removed from the incubator’s environment. Severe changes in the temperature and pH level of the culture medium can have potentially detrimental effects on the embryos. The embryologists are very protective of the embryos in the lab and this is the reason why the embryologists do not like to disturb the embryos too much and will not look at the embryos on a daily basis. Once the embryologist releases the fertilization results, an IVF team nurse will provide an update to you on Day 1 regarding your fertilization results.




2pn Embryo (normally fertilized embryo)




Day 2 through Day 3 – Between days 2-3 the embryos should be dividing through the multicellular stages of development.

 Multicellular Stage Embryos:









Day 4 – On day 4 the embryos should be progressing out of the multicellular stage and should start entering the morula stage. Starting at this stage the embryos rapidly start to divide making it difficult to distinguish the number of cells. Once a morula stage is achieved the embryo begins to move to a cavitating morula phase where a fluid filled cavity forms within the embryo triggering the progression to the blastocyst stage.

 Morula Stage                                                                         Cavitating Morula


cav morula








Day 5 through Day 6 – Between day 5 and day 6 the embryo should continue to rapidly divide and expand into the blastocyst stage. The cells begin to differentiate at this stage determining which cells will become the inner cell mass (potential baby) and which cells will become the trophectoderm (placenta). Day 5 is embryo transfer day! The physician will review the embryology data with you at the time of transfer and will make their recommendations along with the embryologist as to which embryo to transfer. If there are embryos remaining in culture following the embryo transfer and you have consented for embryo cryopreservation, the embryologist will cryopreserve any embryos that reach the appropriate blastocyst stage. Only blastocysts reaching this defined development stage by day 6 will be cryopreserved. Embryos at this defined stage are shown to have higher chances of surviving thawing procedures. After day 6 has passed, and the embryology data has been completed, you will receive an update from an IVF team nurse with whether or not embryos were able to be cryopreserved. Good luck and thank you from the embryologists for allowing us to care for your embryos!

For more information about how RHS grades your embryos, link to our blog, Your Baby’s First Report Card-Embryo Grading.




Blastocyst Stage Embryo




This entry was posted in Conception, In-Vitro Fertilization, Infertility, Reproductive Health, Reproductive Health Specialists, Uncategorized. Bookmark the permalink.