Before the refinement of laboratory techniques, embryos were routinely transferred back into a woman’s uterus within two to three days after egg retrieval (at the 4- to 8-cell stage). It was customary to transfer more than one of these early developing embryos in order to maximize the chance of pregnancy. However, transfer of more than one embryo increases the risk of multiple pregnancies, i.e. twins, triplets or more, which can increase risks to both mother and baby.
Improved Pregnancy Rates
On average, close to one-half of all embryos 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. Only the better quality embryos have the ability to grow to the blastocyst stage and therefore the pregnancy rate per transferred embryo is better for 3-day-old embryos.
In natural conception, fertilized eggs do not enter the uterus until five days after fertilization. Allowing the embryos to reach the blastocyst stage mimics natural conception, and some reports suggest that blastocyst transfer may be helpful for some women with multiple failed cycles. The uterus may be more receptive five days after retrieval because it is contracting less frequently, and is therefore less likely to expel the embryos.
Effect on Women Age 35 and Older
Blastocyst transfer is especially important in women who are 35 years old or older. In this group, it was common practice to transfer 3 or more day 3 embryos.The improved implantation per blastocyst allows the number of embryos transferred to be reduce, preferably to just a single embryo (elective single embryo transfer, eSET). Data indicates that if these women receive only one or two blastocysts, they are as likely to achieve a pregnancy, compared to those who have three or more less-developed embryos transferred. In women under age 35, generally only a single blastocyst should be transferred, otherwise the risk of multiple pregnancies is unacceptable.
Blastocyst transfers increase the likelihood of pregnancy, but there is an increased risk of identical twins. If more than one blastocyst is transferred 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. However, blastocyst transfer also allows the option of single embryo transfer. A single embryo transfer minimizes the chance of twins, which is particularly important 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. 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.