Single embryo transfer and double embryo transfer are both a singular part of fertility treatments. The most common fertility treatment associated with either kind of embryo transfer is in vitro fertilization (IVF), although some other fertility treatments can make use of both variants. When comparing the two variants of embryo transfer, you should look at the similarities, differences and the consequences of choosing one over the other.
The Primary Difference
The primary difference between single embryo transfer and double embryo transfer is, as their names indicate, one embryo versus two embryos. This means that during your fertility treatment, your specialist will transfer only one embryo, or laboratory-fertilized egg, into your uterus during the single embryo variation and two embryos during the double embryo variation. Both are actually significantly less than many variations of embryo transfer, which can call for three, four or even more embryos.
The Advantage of Double Embryo Transfer
The reason why other variations of embryo transfer involve transferring so many embryos is because traditionally, implanting more embryos at once has increased the chance of at least one embryo successfully taking root and leading to a pregnancy. Not all implanted embryos do lead to a successful pregnancy--in fact, until recently, as many as two out of three implanted embryos did not lead to a successful pregnancy.
Because the fertility treatment is costly and puts the woman through a number of surgical (invasive and non-invasive) procedures, it has been thought better to implant numerous embryos at once to increase the likelihood of a successful pregnancy. Although advancements have been made in the fertility field, still not all embryo transfers lead to successful pregnancies. The advantage of double embryo transfer over single embryo transfer, then, is that with two embryos, the chances of a successful pregnancy increases, without having so many embryos implanted that the risk of extreme multiple births (triplets, quadruplets or more) is high.
The Advantage of the Single Embryo Variation
Advances in fertility treatment technology have made it possible to have a significant chance of successful pregnancy with even single embryo transfer. The advantage to choosing single over double or another variation is a smaller risk of a multiple pregnancy--and with that, less risk of the discussion of partial abortion if the number of fetuses is so high it poses a medical risk to you and to your unborn children.
It's important to note that any kind of fertility treatment does have a higher chance of multiple births than traditional pregnancy, including both single embryo and double embryo variations, the former due to splitting embryos. Both the single embryo and the double embryo variations also involve the potential for discarded embryos. Because your fertility specialist will fertilize multiple eggs in the lab and then will select the one or two that seem the most likely to succeed, there's also the issue of scientific selection in the pregnancy process.
Double embryo transfer may have an increased success rate, but single embryo transfer has become increasingly popular due to advances in fertility technology that make it almost as successful as double embryo transfer. Choosing the single embryo variant also significantly decreases your risk for multiple births.