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IVF Embryo Culture to the Blastocyst Stage

A blastocyst is an embryo created during in vitro fertilization that has been grown in the laboratory for five to six days.

Blastocyst Formation

After fertilization has taken place, the embryos are transferred to a special growth fluid (media). The process of allowing embryos to develop in the IVF laboratory is called culturing. In order to enable the embryos to reach the blastocyst stage, the culture dishes containing the embryos and the media are maintained in an incubator where the atmosphere, humidity, and temperature are carefully monitored and controlled.

During this time in the incubator, it is hoped that the embryos will develop normally. Normal embryo development requires a process of cell division called mitosis. The single fertilized egg divides into two cells, then four and so on. Some embryos will not divide at all. Some embryos may divide at a very slow rate which is indicative of a poor potential for producing pregnancy. Some embryos may divide abnormally and start to fragment during their development. These embryos also have a poor potential for producing pregnancy. Normally, by the third day after the egg retrieval, healthy eggs will have divided into eight cell embryos.

Stages of embryo development

Embryos that have only divided into 2 or 4 cell embryos by the third day have a lower potential for producing a pregnancy. Sometimes, the embryos don’t divide equally giving rise to embryos with three, five or seven cells.

In addition to looking at the number of cells, embryologists will also grade the embryo by looking at its appearance under the microscope. Embryos that are dark in color or that have extensive fragmentation will also have a lower potential for producing pregnancy.

A “perfect” embryo would get a score of 1. A poor embryo might have a score of 3 or 4. We would like to see embryos that are 8 cells and grade 1. A patient may have a lot of good embryos or she may have none. Unfortunately, this scoring system isn’t perfect for predicting which embryos may produce a pregnancy. For this reason, some IVF programs may place more embryos into the uterus at this stage in order to increase the chances for a pregnancy. Unfortunately, this increases the risk for having a multiple pregnancy.

New technology allows for time lapse imaging of embryo development. Sophisticated software can take measurements of the exact timing of certain events. It is hoped that someday, as this technology improves, that it will enable selection of the best embryos for transfer. For now, however, time lapse imaging does not do better than an experienced embryologists.

Blastocyst Culture

Transferring embryos at the blastocyst stage will maintain the highest chance for pregnancy while keeping the risk for multiple pregnancy low. Blastocyst development requires culturing the embryos for two additional days in the laboratory. By this time (day 5 after the egg retrieval), healthy embryos will have reached the blastocyst stage.

Blastocyst Development

By definition, a blastocyst is an embryo that has divided into hundreds of cells and is composed of two parts. The outer sphere of the blastocyst is called the trophoblast. The inner portion of the blastocyst is filled with fluid. Inside the trophoblast, there is a clump of cells called the inner cell mass. The inner cell mass is the portion of the blastocyst that actually becomes the baby.

Blastocyst Transfer

Only a small percentage of embryos will reach the blastocyst stage. This self selection of the embryos allows the embryologist to pick the embryos that are most likely to produce a pregnancy. When transferrring blastocysts, it is not necessary to transfer as many embryos to maintain a high pregnancy rate. This reduces the risk of multiple pregnancy and therefore increases the chances for a couple to have a live born healthy baby.

Many experts believe there is another reason for the higher pregnancy rates seen with blastocyst transfer. In a normal “in-vivo” (in the body) conception, the embryo does not reach the uterus until the blastocyst stage. If we were to place an embryo into the uterus at the 8-cell stage (after only three days of embryo culture, it is deviating from the normal body processes. That is, the 8 cell embryo does not belong in the uterus at that time. Transferring the embryos at the blastocyst stage is more “physiologic”.

Studies now demonstrate conclusively that the chance for pregnancy and the risk for multiple pregnancy are reduced with the use of blastocyst transfer. This is true for all age groups.