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Saturday, January 19th 2008, 12:06pm


Blastocyst Culture

Blastocyst culture and transfer is a new important technique developed for in vitro fertilization (IVF) that maximizes pregnancy rates while minimizing the risk of multiple pregnancies. The ability to grow embryos for five days to the blastocyst stage of development in the laboratory, rather than the traditional two to three days, allows embryologists to determine with greater certainty which embryos are really the "best" in terms of their potential for implantation. It has taken many years of research to finally develop the right culture medium for blastocyst development.

What is a Blastocyst?

After egg retrieval, the fertilized egg may develop into an embryo over a period of three days in the laboratory. The embryo typically divides into between four and ten cells. A blastocyst, by comparison, is an embryo that has advanced to the five-day stage; which means the embryo has divided many more times into many more cells over the five-day period following an egg retrieval.

Blastocysts have a very thin outer shell and do not require assisted hatching, thus potentially increasing the odds of implantation into the uterine cavity. While the majority of fertilized eggs will develop into a three-day old embryo, only perhaps 40% of these embryos will develop into a blastocyst. Therefore, blastocysts are considered to be a more "select" group of embryos with a higher chance of pregnancy.

What are the Advantages of Blastocyst Culture?

Perhaps the best way to show this is to give an example:

If a woman has 15 eggs retrieved, an average of 10 will fertilise the following day. It is impossible to determine at this point which of these 10 are most likely to implant and develop into a baby. On the traditional day of embryo transfer (day two or three); perhaps five of the 10 embryos may be developing into good quality normal embryos. The others may have slowed or stopped their development altogether. As we are limited by the HFEA to transferring a maximum of two embryos,(three embryos if over 40), it can be difficult to select the one embryo that may develop into a baby (if any).

All five embryos may look like they have the same excellent potential. Two additional days in the blastocyst culture medium allows the natural selection process to continue. Thus, after five days of growth in the laboratory, only one, two or three of the original ten embryos may remain viable. We now know the best embryos to transfer.

In fact, pregnancy rates in the group using blastocyst transfer may even be higher because the uterine lining is more receptive to the advanced embryos. In addition, if there are more than two available at the time of transfer, the remaining blastocysts may be cryopreserved for future use.

A significant benefit of blastocyst culture and transfer is the reduction of multiple births that can result from IVF. This means that various obstetrical complications that may arise from multiple pregnancies can be minimized. It is especially important in helping patients avoid having to make the difficult personal and ethical decisions regarding selective reduction. You must be aware, however, that a blastocyst can divide into an identical twin after it is transferred to the uterus, and so there is still a very small possibility that two blastocysts can result in a triplet pregnancy.

What are the Disadvantages of Blastocyst Transfer?

As stated above, it is more difficult for an embryo to develop into a blastocyst. Therefore, there is the possibility that an IVF cycle will not result in a transfer of any embryos if none of the three-day old embryos develop into blastocysts. We believe that if blastocyst development does not occur, it is highly likely that a pregnancy would not have developed if the embryos had been transferred at the two or three day old stage. Nevertheless, it may be difficult to accept having gone through an IVF treatment cycle and not having the chance of becoming pregnant through the transfer of any type of embryo.

We can view this, however, as a possible advantage of blastocyst transfer, if the embryos are not going to develop into blastocysts, then it is unlikely the transfer of earlier stage embryos would have achieved pregnancy. The time, expense, and the waiting for two weeks to find out if a pregnancy had occurred can be eliminated.

Another disadvantage of Blastocyst culture is the poor success rates associated with blastocyst cryopreservation. Some clinics have reported results of blastocyst freezing with resultant clinical pregnancy rates of up to 30%. However, most units worldwide have reported results much lower than this (less than 10%), which is probably a more realistic figure. Certainly, one of the biggest dependent variables in this situation is the quality and number of blastocysts available for cryopreservation.

Is Blastocyst Transfer for me?

Many clinics are recommending blastocyst transfer to those patients who have a better chance of having blastocyst development (i.e. women less than 37 years of age with a reasonable number of good quality embryos) with two previous failed attempts at achieving a pregnancy. This is despite having high numbers of good quality embryos available for transfer.

Patients who have fewer oocytes retrieved, fewer fertilised or fewer dividing embryos by day three in culture have no advantage using blastocyst culture, since little is to be gained in further embryo "self selection". Unfortunately the new blastocyst culture media does not improve the health or viability of an individual embryo that is not otherwise able to sustain five days of growth and then implant. Rather, it allows embryos capable of sustained growth to continue in culture and reach their maximum inherent capability.

Blastocyst transfer is also considered for those patients who have strong concerns about delivering high order multiple pregnancies, and for those who would not undergo fetal reduction.

Blastocyst transfer is an excellent choice for women who undergo a frozen embryo transfer cycle. The embryologists can thaw most or all of the frozen embryos and see which ones develop to the blastocyst stage (which, again, would be inherently more likely to implant and result in a successful pregnancy). It is important to consult with your clinician about whether blastocyst culture and transfer is for you.