The Life of an Egg – 3 of 3

Days Three to Seven

Original article, The First Day
Continued from The Second and Third Days

If we leave pre-embryos in culture for two or three more days, some of them will reach the next stage of development called the blastocyst. A blastocyst has about 30-100 cells and the cells have become specialized. A big fluid-filled cavity forms inside the mass of cells and pushes the cells to the outside. The outermost ring of cells is called the Trophectoderm and will eventually become the placenta. An additional group of cells will be pushed to one spot beside the Trophectoderm. This layer is called the Inner Cell Mass and will eventually become the baby. Once the cells of the Inner Cell Mass begin to develop independently of the trophoblast then this group of cells is now called the embryo.

Sometimes we transfer blastocysts. If we have already transferred and/or frozen multicell embryos we still culture anything we have left to see if they become blastocysts. If they become good blastocysts with both an Inner Cell Mass and a nice Trophectoderm we can freeze these embryos too. However, poor quality Day 3 embryos are not as likely to become blastocysts as the embryos we usually pick to transfer on Day 3, or the ones we freeze on Day 3.

We do not believe that the remaining embryos we keep in culture through Day 7 which do not become blastocysts should be frozen. Usually it is obvious by this time that the embryo is no longer dividing, and no longer capable of becoming a baby. There is also a lot of information to suggest that most of these poor quality embryos are genetically abnormal. Freezing abnormal or poorly dividing embryos will not improve the chances of becoming pregnant in future cycles.

Therefore, some patients will be able to have embryos frozen on Day 3 and/or Day 5, 6, or 7. However, some patients will only get enough embryos for the fresh transfer. We have no way to guarantee to any patient that they will get embryos to freeze for future cycles.