Early days in the lab
Once an egg is successfully removed during IVF and combined with sperm, a single-cell embryo is created. That single cell then divides in the lab every 12-24 hours, growing bigger and stronger. On Day 3, a normally growing embryo will have 6-10 cells and can be transferred back into the uterus. Some patients prefer to wait until Day 5, when the embryo consists of 70-100 cells and is now called a blastocyst.
Day 3 proponents
Fertility clinics have historically transferred embryos on Day 3. The chances of success were high enough that waiting for the embryo to divide further was thought unnecessary. However, research shows that Day 5 embryos have a higher implantation and pregnancy rate. There are still some situations where a woman might benefit from a Day 3 transfer as when there are fewer good-quality embryos to select. In this case, the selected embryo may benefit from being placed in the uterus earlier. The woman can provide a better environment for the embryo than the lab dish and continued growth is possible.
Risk of multiples
Implantation rates are expected to be lower with Day 3 embryos, and estimates of success are 10-20%. For that reason, some doctors prefer to transfer more than one embryo to improve the chance of pregnancy. While twins or triplets may be appealing to some patients, multiples carry risks. Women who are pregnant with multiples are more likely to experience miscarriage, deliver preterm, develop gestational diabetes, and experience postpartum hemorrhage. Carrying a single baby is a much safer process.
Day 5 embryos
By waiting a few additional days, the fertility clinic can take a closer look at the embryos and determine the best one to transfer. Day 5 embryos are also able to be genetically tested, reducing the risk of transferring one with a chromosomal abnormality. Day 5 embryos are more mature and are thought to be stronger and more likely to result in pregnancy. Some patients prefer transferring during Days 5-7 to mimic what happens in a natural pregnancy when the embryo implants.
Although both Day 3 and Day 5 embryos have a shot at success, there may be a slight advantage to choosing a Day 5 transfer. However, the best way to know for sure is to ask the fertility specialist. The doctor will assess pregnancy history, embryo quality, and desire for genetic testing to determine the best transfer day. Whatever option is chosen, the IVF procedure is a great way to build a beautiful family.