Day 3, Day 5, Day 6 Embryos? What is the difference and why does it matter when looking for Donated Embryos

 

There are many important questions to ask a potential Embryo Donor when you're considering building your family with Donated Embryos. This full list can be viewed from the link below. 

https://embryologistonfire.blogspot.com/2023/01/what-to-ask-when-looking-for-donated.html)

The question for today is...

On what Culture Day was the embryo cryopreserved (frozen)?

 Cryopreservation can take place on different days for different reasons. 

Day 0: 

When a human egg is retrieved at the retrieval, this is day 0. The eggs are either cryopreserved (frozen as eggs) or fertilized in the laboratory either by ICSI (intracytoplasmic sperm injection) or conventional insemination on this day.  Cryopreservation on Day 0 is unfertilized oocyte (egg) freezing. This is done for fertility preservation and more commonly for availability of frozen unfertilized donor eggs through a clinic or donor egg bank. 

Day 1:

Most laboratory teams will check for fertilization 14-18 hours after conventional insemination or ICSI, on Day 1. A normally fertilized oocyte has identifiable structures called pronuclei (2) and polar bodies (2) and is commonly referred to as a "2PN". Cryopreservation of the 2PN is not common but may be performed due to laboratory problems, timing/laboratory batching, or a desire to ship and culture the embryos at a different location. Abnormal fertilization can occur, but these are rarely cryopreserved.

Day 2:

On Day 2 the embryo is typically between 2 and 4 cells. The cells are easily counted, and a good quality embryo has clear fluid around the cells, with no cellular debris. While Day 2 embryos can be cryopreserved, it is uncommon practice to cryopreserve them on this day. 

Day 3:

Day 3 embryos are expected to be between 6 and 8 cells, which is desired. Is it not uncommon for day three embryos to be 5 cells, or greater than 8 cells, but these are typically selected after those showing 6 or 8 cells. Because many embryos arrest or slow in development between day 3 and 5, the advantage of longer culture is identifying and removing those embryos as candidates for transfer. 

Day 3 is becoming an uncommon day of cryopreservation but was seen more often in the early 2000's. While it is now standard in most programs to culture embryos to day 5 or 6, Day 3 cryopreservation can be advantageous for patients who are advanced maternal age (typically women >38), when a low number of embryos are available, or if it is the clinic protocol to transfer/cryopreserve embryos on this day. 

Day 4

On Day 4, the embryo reaches the Morula stage and is rarely cryopreserved. It becomes developmentally polarized and begins to form the ICM (inner cell mass) and (TE) trophectoderm layers. 

Day 5

Day 5 is when the embryo is expected to reach the blastocyst stage, many times referred to as a Blast. The blastocyst stage is the most common day for cryopreservation, and most embryos survive the freeze/thaw process with ease. (The cover photo is a Day 5 embryo)

Day 6 

Day 6 is also a very common day for cryopreservation. While the Day 6 embryo is considered inferior to a day 5 embryo of the same grade, they are not considered compromised in most programs.  It is very likely to have a higher number of day 6 embryos than day 5 embryos when embryo biopsy is performed. Many times, even if an embryo is eligible for cryopreservation on day 5, it is left in culture to increase the cell number and maturity/expansion for a safer and more efficient biopsy procedure. 

Day 7

While some clinics will keep an embryo in culture and cryopreserve on day 7, it is not common. A day 7 embryo would be selected last in most cases, if cryopreserved. It is common for embryos to be discarded if not eligible for transfer or cryopreservation by day 6.