Is the success rate higher with fresh or frozen embryos ?

In the past, everyone agreed that the success rate was much higher with fresh embryos. This was primarily because the embryo freezing technology was not very good. We used a technique called slow freezing, and unfortunately, as a result of the freezing and thawing, upto 50% of embryos ( or the cells in the embryo) would die. This is because embryo cells ( called blastomeres) contain a lot of water; and the freezing process would lead to ice crystal formation within these cells. When these embryos were thawed, these crystals would cause cell death. Obviously, embryos will dead cells have a much poorer chance of implanting, and most labs were resigned to this fact that the process of freezing and thawing would cause damage to some embryos. However, it was still well worth freezing, because the success rate for the embryos which survived the insult of the slow freezing and thawing had excellent implantation rates !

The good news is that the slow freezing technology has now been replaced by ultra-rapid freezing. This is called vitrification, and is the only freezing process we use routinely in our lab today. We have stopped slow freezing for the last 2 years, because the success rates with vitrification are so much better.

In fact, when vitrification is done by an expert, the survival rate is nearly 100% - which means practically all the cells in the embryo survive the freezing and thawing intact.

This is a huge advance, because we can now freeze supernumerary embryos routinely, and we are confident that they will survive the thaw !

Vitrification is a tricky process and does have a steep learning curve, which means that not all labs will have a good success rate with it. This is why some labs still use the older slow freezing technique, because they have more experience with it, and are skilled at doing this. It's sometimes hard to teach an old dog new tricks !

Since the survival rate of embryos after vitrification is so good, we have found that our success rate after transferring these frozen embryos is often better than with fresh embryos transfer. This may seem quite counter-intuitive ! After all, if we are transferring the top embryos in the fresh cycle, and freezing only the supernumerary embryos ( which are presumably not of as good quality), how could this be possible ?

There are 2 reasons for this.

1. The number of fresh embryos we transfer has now been cut down to 2-3 ( or even one, when we are transferring blastocysts

2. The uterine lining is far more receptive in a non-IVF cycle. During IVF, the meds we use for superovulation can cause the uterine lining to be out of synch with the embryo. In a frozen cycle, we can prepare the uterine lining, till it's maximally receptive for implantation !

This means that it's possible that IVF clinics will soon start moving towards doing more frozen embryo transfer cycles than fresh cycles ! ( In fact, the Kato clinic in Japan which does about 10000 cycles per year) does not transfer any embryos at all in the fresh cycle ! They electively vitrify all the blastocysts and transfer in the subsequent cycle - and they have an extremely high pregnancy rate.

What does all this mean for you ?

1. Before you start your IVF cycle, confirm that your clinic uses vitrification for freezing embryos; and they are good at this
2. Don't ask your doctor to transfer more than 1-2 embryos
3. Be proactive about freezing your spare embryos
4. Even if you do not get pregnant in your fresh cycle, do not lose heart. The chances of success with vitrified embryos is still excellent !