Embryo Development

During the first five days after fertilization, an embryo rapidly passes through multiple stages:

• On day one, an embryo is typically at the pronuclear stage, commonly referred to as 2PN. It is still a one-cell embryo, because the DNA material from the sperm and the egg has not fused yet.
• Day two after fertilization, the embryo is at the two- to four-cell stage.
• On day three, it is typically at the six- to eight-cell stage. Days two and three are also called the cleavage stage.
• On day four, the embryo is at the morula stage. A morula is a solid sphere that has many cells.
• By day five, most embryos have reached the blastocyst stage. The blastocyst stage is the first stage of embryo development in which the different cells are differentiating (developing different functions). A blastocyst embryo has three components:
  • An inner cell mass, which ultimately becomes the actual baby
  • An outer cell mass, which becomes the placenta
  • A fluid-filled space, the blastocoel

Embryo transfers are typically performed on day three (the cleavage stage) or on day five (the blastocyst stage).

Advantages of Blastocyst Transfer
Studies have shown that day-three transfers and day-five transfers have comparable pregnancy rates; however, culturing embryos until day five, whenever possible, allows for the selection of the best embryos and the transfer of fewer embryos, which decreases the risk of multiple pregnancies while maintaining high pregnancy rates.

One of the distinguishing qualities of the IVF Institute is the quality of our IVF laboratories. Our laboratories have excellent “blastulation” rates. This means that, with very few exceptions, we are capable of successfully culturing embryos to the blastocyst stage and therefore can take full advantage of performing a day-five transfer. In addition, any excess high-quality embryos on day five that are continuing to develop will be cryopreserved (frozen), which allows them to be used in the future if desired.

Although there are several advantages to blastocyst transfer, a day-three transfer may be more advantageous for some patients. One of our infertility specialists can advise you as to whether you are a suitable candidate for blastocyst transfer.