Multiple pregnancy is associated with certain increased risks for both mother and fetus. Many couples would prefer to avoid the risk of multiple pregnancy, but find themselves in a dilemma because they also want a high probability of success.
In IVF, multiple embryos are normally transferred in order to achieve acceptable success rates. Back in the days when implantation rates were 5% or 10%, it was common to transfer 3 or more embryos in hopes of getting at least one implantation.
Of course, the transfer of many embryos incurs the risk that many will implant.
The key to reducing the risks of multiple pregnancy is to increase the implantation rate (the proportion of transferred embryos that implant in the uterine wall) so that a good rate of success can be obtained with fewer transferred embryos. Ideally, just one embryo would be transferred in couples seeking to avoid the risk of twin pregnancy.
According to the Society for Assisted Reproductive Technologies (SART) report for 2008, The Fertility Center of Las Vegas achieved an implantation rate of more than 50% in fresh non-donor cycles in patients less than 35 years of age. This means that more than half of the embryos we transferred resulted in a fetus with an observed fetal heartbeat.
However, we can do better than that. We and others have have found that the uterine environment is compromised by ovarian stimulation, making it less receptive in fresh non-donor cycles. To get around this, we can freeze the entire cohort of embryos. In these dedicated freeze-thaw cycles, we have found the implantation rate to exceed 60% per transferred embryo in patients less than 35 years of age. That is, almost 2/3 of transferred embryos resulted in a fetus with a fetal heartbeat.
These rates were achieved without genetic screening of the embryos. With genetic screening, it appears to be possible to achieve even greater implantation rates.
With these high implantation rates, the transfer of a single embryo is a very appealing and successful proposition.