The majority of fertilized eggs can make it to Day 3… the embryo is coasting, so to speak, following the instructions inherited by the sperm and egg. It’s beyond day 3 that the embryo has to erase these instructions and start off fresh and turn on its own genes which then direct development to the day 5-7 stage (the blastocyst stage). The blastocyst stage allows for the embryo to be safely biopsied for genetic testing. It also allows for distinction between embryos that aren’t healthy enough to keep growing versus those that make it and therefore inherently have better reproductive potential.
If one conceived from an ovulated egg, the fertilized egg growing into a day 3 embryo while still in the fallopian tube… it is only when it becomes a day 4-5 embryo that is arrives at the uterine cavity and then it further develops into a blastocyst and hatches from its shell and implants. So it is more synchronous with the uterine cavity environment and what normally happens on its own, to put back a day 5-7 embryo during transfer versus an earlier stage day 3 embryo.

Only ~60% of day 3 embryos turn into day 5-7 embryos - and this depends on mainly egg/embryo quality (but also the lab environment - some are more confident at growing embryos out to day 5 than others). If day 3 transfer is done, this is why multiple embryos are always transferred to make up for the fact that their pre transfer probability of being able to continue growing is lower than a blastocyst embryo. Therefore, there is also higher rate of twin/multiple pregnancy.

For this reason, #blastocyst transfers have become the standard in our field over the past decade.

What questions do you have?

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