Why Do We Prefer to Transfer Embryos at the Blastocyst Stage?

For the first 10 years following the birth of the first IVF baby in 1978, all embryos were transferred two or three days following fertilization. Why? At the time, embryo culture was limited by the simplicity of the available culture media which would not allow growth in the incubator after the third day. Embryo development

In nature, fertilization takes place inside a woman, or “in vivo,” at the end of the Fallopian tube where the embryo spends the first three or four days of its life. Then it travels for one day, covering a distance of two to three inches, before being expelled into the uterine cavity in the form of a blastocyst that will soon hatch and implant itself in its biological nest.

This meant that the first IVF pioneers had only one option, to place the two or three day embryo in the uterus and hope that Mother Nature would be kind enough to let it grow undisturbed until nesting time arrived. Allowing the embryos to reach the fifth day or blastocyst stage made sense. But, how could we achieve that goal?

Day 3 8 cell embryoScientists first developed a culture system called “co-culture” which would provide the essential nutriments to overcome this hurdle. It consisted of a carpet of “feeder” or “helper” cells designed to remove toxins and add growth factors. This system allowed scientists to discover the necessary missing elements and a more complex medium was developed. Nowadays, early embryos are placed in this new and richer environment on day 3, giving them the ability to reach the blastocyst stage. There is a big difference between a third day, with 8 to 10 cells, embryo and a day 5 blastocyst which contains between 150 and 220 cells. In the blastocyst, you can detect the inner cell mass, which is actually the future fetus and the trophectoderm, which is the future placenta. Not only is placement of the embryo at the blastocyst stage more natural, but we can better select the embryo that will have the best chance of survival. In addition, and embryo is easier to freeze or biopsy at the blastocyst stage.

BlastocystWe still consider that there are indications for day 3 transfers. On occasion, there are reasons to feel that certain early embryos are not thriving in culture and we must transfer them early under the presumption that they might have a better chance to survive in the maternal incubator. Interestingly, I recently read that more than 50% of IVF centers in this country are still transferring embryos only on day 2 or 3. The reasons given are:

  1. “Less working hours in the lab, saving time and overhead.”
  2. “Lack of trust in the culture media or incubators.”
  3. “Preference for the maternal incubator, which means the uterus.”

At SMFI, whenever possible, we transfer at the blastocyst stage, meaning about 90% of the time. Our embryologists work hard and long hours. They do not hesitate to sacrifice time for better results.