When a couple undertakes an IVF cycle we usually attain say up to a dozen eggs and following fertilisation and culture, there might ultimately be in the region of 1 – 4 applicable embryos at the day 5 stage. At that point, our usual advice is to transfer the best blastocyst and freeze the remaining blastocysts for later use.
There is an erroneous view that transferring two embryos fresh will somehow lead to a higher chance of having a baby, compared to transferring one. However at Sydney IVF, our freezing technology for embryos (using a rapid freezing technique called vitrification) leads to the same chance of a vitrified embryo implanting in a subsequent cycle as the chance would be if that individual embryo had instead been transferred fresh. In other words, if a couple have two good quality blastocysts, the chance of becoming pregnant is the same whether one embryo is transferred fresh and (if necessary) the second embryo is transferred later in a frozen cycle; or alternatively whether two are transferred together in the immediate fresh cycle. In other words, 1 + 1 is the same chance of getting pregnant as 2 at once!
The only difference down either pathway is the risk of twins. Transferring two embryos fresh means that if you get pregnant, around one third of those pregnancies will be twins. And if you think this is the answer to your dreams, think again. Apart from the social challenges of raising two babies at once (e.g. the lack of sleep for 2 years), there are far higher rates of obstetric complications with twins. These include a higher chance of prematurity, abnormalities including cerebral palsy, developmental delay and death in twins, compared to singleton pregnancies.
In summary, at Sydney IVF, you have a higher chance of having a healthy baby if you transfer one fresh and one frozen compared to transferring two fresh. The same principles apply when there are multiple embryos frozen. Couples often ask whether they should transfer two frozen embryos in a cycle, compared with transferring one frozen embryo each time, in different cycles. The ultimate chance of having a baby from the use of frozen embryos is the same, whether they are transferred one at a time or transferred two at a time. There is certainly no evidence that they somehow positively influence each other to implant if placed together in the uterus.
The decision to transfer one or two frozen embryos in one cycle, then becomes more an issue of a short term saving in cost and time, traded off against a risk of twins.
Putting all of this together, if a couple ask me whether one or two embryos should be transferred, my response from a purely scientific prospective would usually be one – because transferring one at a time will NOT lower your ultimate chance of getting pregnant, but WILL tend to avoid the elevated risks that twins bring.