Embryo Development and Cryopreservation

Whether you are currently going through a cycle of in vitro fertilization (IVF) treatment or you have had unsuccessful IVF treatment in the past, it is important for you to be able to understand the quality and development of your embryos. After reading this fact sheet you will have a better idea of what questions to ask and understanding of the information that you will get.

After your eggs have been fertilized by either conventional IVF fertilization or intra-cytoplasmic sperm injection (ICSI) you have to wait until the next morning in order to find out how many eggs fertilized. Your fertilized eggs (embryos) are then assessed by a scientist on a daily basis until your embryo transfer.

The scientist will talk to you about the physical appearances (morphology) of your embryo(s) including the number of cells that are present and if there is any fragmentation in the embryos. These physical characteristics help scientists to determine the quality of your embryo(s).

Embryo Cell Number

The first cell division is evident on day 2, when embryos should be at the 2-4 cell stage.  Embryos continue to develop, and should ideally be at the 6-8 cells stage on day 3, and the morula stage on day 4. Morula means mulberry and is a cluster of cells that is starting to compact into a solid mass which is the developmental stage before blastocyst formation.  

Whilst these are the normal milestones for the development of good quality embryos, it is common for the progress of some of the embryos to slow down or stop during the 5 days of culture in the laboratory. We grow the embryos for 5 days in order to closely monitor their development and so select the best quality embryo(s) for transfer and freezing (cryopreservation). 

Embryo Fragmentation

When the embryo cells divide during development, small pieces can sometimes break off. This is described as fragmentation. Each embryo is assessed as having nil, mild, moderate or severe fragmentation based on the percentage of fragments contained within the embryo. Embryos with severe fragmentation will sometimes have a lower implantation (pregnancy) rate. However, some embryos have the capacity to develop normally despite such fragments as they develop into a blastocyst. Most embryos exhibit some degree of fragmentation and there are plenty of babies born from embryos that were fragmented at the time of transfer. There is no evidence that the degree of fragmentation is linked to the chance of a baby being born with abnormalities.

Blastocyst Stage Development

A blastocyst is an embryo that has developed to the point of having 2 different cell components and a fluid cavity. Human embryos usually reach the blastocyst stage by day 5 after fertilization. As the development of a blastocyst progresses, the cells in each of the 2 components divide and the fluid cavity enlarges. With continued blastocyst development the embryo expands, the shell thins, and the blastocyst hatches out of its shell. Blastocyst implantation into the lining of the uterus can then begin. A healthy blastocyst will implant within about one to four days following a day 5 embryo transfer, with invasion of the cells into the uterine lining occurring soon after blastocyst hatching. Transferring a single blastocyst during IVF treatment can give a high pregnancy success rate with a very low risk of triplets.

Blastocysts can be graded on their quality based upon three criteria;

A.  Blastocyst development stage - expansion and hatching status

 
B.  Inner cell mass (ICM) score, or quality 

 
C.  Trophectoderm (TE) score, or quality

Cryopreservation (Freezing) 

Not all excess embryos (those not immediately required for transfer) are good enough quality for freezing, with on average of 30-40% of excess embryos being suitable for freezing on Day 5. Occasionally embryo development will be a little delayed and the blastocyst stage will not be evident until Day 6. Therefore, we sometimes incubate excess embryo(s) for an extra day beyond the transfer. If they reach the blastocyst stage on day 6 then they are still suitable for freezing, but we do not culture embryos any longer than 6 days

Embryo freezing is undertaken by a technique called vitrification, whereby a single embryo is suspended in a tiny drop of special freezing fluid and snap-frozen.  Embryos are then stored in a straw at -196 degrees centigrade in liquid nitrogen. Each embryo straw is labeled with your full name, date of birth, a unique identification number and the date of freezing. 

Not all embryos will survive the freeze-thaw process, with on average 85% of embryos surviving and being suitable for transfer in the future. Unfortunately there are occasions where no embryos will survive. 

As per Australian government protocols, embryos can be stored for up to 5 years, after which approval from the clinic must be sought for an extension of storage. Please remember to advise the clinic immediately of any change of address or circumstances if you have frozen embryos in storage. Storage accounts are sent out every 6 months. 

A copy of our Embryo Development and Cryopreservation Fact Sheet can be downloaded from the bottom of this page.

• Embryo Development and Cryopreservation Fact Sheet - 109 KB

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