Typically, embryos from in vitro fertilization had been transferred back into a woman's uterus at the 4- to 8-cell stage two to three days after egg retrieval. It has been customary to transfer more than 1 of these early developing embryos in order to maximize the chance of pregnancy. Unfortunately, this practice also increases the risk of multiple pregnancies, i.e. twins, triplets or more.
On average, close to one-half of 4- to 8-cell embryos at day 2 or 3 are chromosomally abnormal. By culturing these embryos for an additional 2 to 3 days, more chromosomally normal embryos will be selected out naturally and develop to the blastocyst (5-day-old) stage in the laboratory. Usually, only the best embryos have the ability to grow to the blastocyst stage and the pregnancy rate per blastocyst transferred is better than 3-day-old embryos. This allows the number of embryos transferred to be reduced. This is especially important in women who are 35 years old or older because in this group of women, it had been common practice to transfer 3 or more day 3 embryos. This practice is associated with an increased risk for triplets. Data indicate that if these women receive only two blastocysts, they are as likely, or even more likely to achieve a pregnancy, compared to those who have three or more less-developed embryos transferred. Even in women under age 35, transferring two blastocysts at day 5 may lead to a higher chance of pregnancy than transfer of two embryos at day 3.
There is an increased risk of identical twins with blastocyst transfers. When this occurs, there is the possibility of triplets, where both blastocysts implant and one of those blastocysts splits. Identical twins have an increased chance of obstetrical complications compared to non-identical twins.
Another advantage of developing all embryos to the blastocyst stage is that it makes embryo cryopreservation (freezing) more efficient, since embryos without the capacity for full development are not frozen.
Blastocyst transfer also allows the option of single embryo transfer. This is particularly helpful to reduce the chance of twins when twins would be more risky, such as with a misshapen uterus due to certain uterine abnormalities. Any couple wishing to avoid the chance of twins for any reason may choose single blastocyst transfer.
In natural conception, fertilized eggs do not enter the uterus until five days after fertilization. Some reports suggest that blastocyst transfer may be helpful for some women with multiple failed cycles, since the uterus may be more receptive five days after retrieval and because the uterus is contracting less frequently, therefore being less likely to expel the blastocysts.