During a typical IVF round, embryos are developed in a lab for two to three days under extremely controlled conditions. Then, any extra embryos can be preserved for later use after being implanted into the woman’s uterus (womb). The term “blastocyst culture” refers to the embryos being grown in the lab for two more days after which they are known as blastocyst embryos. Blastocyst transfer simply refers to the day 5 transfer of blastocyst embryos to the woman’s uterus in the same manner as day 3 embryos.

Simply said, the “survival of the fittest” principle can be applied to this self-selection process. The embryos that make it to this stage of growth are more likely to be robust, strong, and healthy. The embryologist may advise transferring the embryos on Day 3 or delaying it until Day 5 depending on the quantity and quality of the embryos on Day 3 and the patient’s medical history. Patients who have fewer retrieved oocytes, less fertilized, fewer dividing, or fewer high-quality embryos on day three do not benefit from blastocyst culture since there is not much to be gained through additional embryo “self-selection.”

During the early stages of IVF, doctors frequently transferred numerous embryos into the uterus to accommodate the unsuccessful ones, which caused multiple births in some women. Most importantly, because we can choose our embryos carefully using blastocyst culture, we can confidently transfer fewer, frequently single embryos and prevent multiple pregnancies.