One of the most vexing problems encountered during fertility treatment is how best to optimize the pregnancy rate will we also minimize the risk of triplet (and higher) pregnancies. Since we can’t currently tell which egg has the greatest potential to become a healthy baby, the most common strategy used by IVF centers is to produce as many eggs as possible. Attempts are then made to fertilize each one and let them grow to identify those that are most likely to be capable of implanting and becoming a healthy baby. Generally speaking, around 10-30% of eggs have the ability to complete this initial development in the lab. For those couples fortunate enough to have more than one or two healthy embryos; the question arises about what to do with the rest. Transfer them and risk experiencing a multiple pregnancy or freeze them?
The first pregnancies resulting from frozen and thawed embryos were delivered in 1984. Since that time, hundreds of thousands more have followed. In 2009 for instance—the most recent year with statistics available[r1] —there were nearly 22,000 frozen embryo transfers in the USA and about a third resulted in a successful pregnancy. It’s also noteworthy that once frozen, embryos retain their potential to result in the birth of a healthy child. A report in 2010 [r2] demonstrated this when a couple that conceived through IVF in 1990, donated their remaining embryos and one implanted resulted in the birth of a healthy baby after being frozen for nearly 20 years. One large European study [r3] found that pregnancies conceived with frozen embryos may have a lower risk of medical complications than those resulting from a fresh embryo transfer. So although cryopreservation is not fail-safe, it is viable option to reduce the risk of multiple births. The most common question is how many embryos survive the process of freezing and thawing.
Human embryos are at risk of thermal injury if not frozen carefully. Initially, they were typically frozen at a controlled rate while a substance called a cryoprotectant was added to reduce damaging ice crystal formation—a process now referred to as the “slow freeze” or “conventional” method. Once frozen the embryos are stored in liquid nitrogen at a temperature of -196⁰C. Typically, most centers report that about 70% of the embryos survive the freezing and re-warming process using this technique.
More recently, many centers have adopted a new and very rapid freezing technique called “vitrification.” The advantage of this new rapid freeze technique is it occurs too rapidly for ice crystals to form therefore reduces the risk to the embryos. As a result, very few healthy embryos are damaged through this process. In fact, this technique is now being applied to freezing unfertilized eggs as well; an even greater challenge than freezing embryos. Several recent analyses published in the August issue of the journal Fertility & Sterility[r4] predict that this technique will rapidly become the standard although currently not all clinics have switched to vitrification at this time.
So when considering how many embryos you wish to receive in an embryo transfer, consider the risk of multiple pregnancy as well as the success of frozen embryo transfer. It’s worthwhile to talk to your clinic about their freezing technique and their experience. It could make a world of difference you and your family.