Which Freezing method in IVF has the best Success Rate?

There have been several studies published over the past few years that scrutinize and explore whether the transfer of fresh or frozen embryo results in higher success rates in IVF pregnancy. Also, there have been various studies on which transfer type is associated with the healthier development for babies and mothers.

In recent times, most IVF programs have stopped fresh embryo transfers and are exclusively executing frozen embryo transfers. Different IVF agencies in the United States have also slowly moved towards more frozen embryo transfer than fresh transfers. This might become a long-term trend to help couples achieve the most successful outcomes of surrogacy treatment.

There are two common issues related to the superiority of frozen embryo transfer in comparison to fresh transfer.

Embryos grown healthily in the laboratory for a span of 5 to 6 days are known as blastocysts. Blastocysts are naturally made of 100 to 230 cells on an approximate. Most IVF treatment agencies prefer to freeze blastocysts instead of 1, 2, 4, 6 or 8-cell stage embryos.

Advantages of freezing blastocysts are the followings:

  • It allows selecting better embryos before the transfer.
  • It reduces the possibilities of multiple gestations.
  • The transfer of embryos occurs when the time is closer to the natural time of uterine implantation and it is almost ready to hatch. Sometimes hatching also takes place at the time of embryo transfer.
  • Blastocyst freezing is more definitive and dependable than the earlier practice of freezing of day 1, day 2 or day 3 embryos. It provides a much higher success rate of live birth for every transferred embryo.

The success rate of implantation could possibly get decreased in fresh transfers in comparison to the frozen embryo transfers. It is because the frozen embryo transfer provides a “healthier” implantation process in the frozen embryo transfer (FET) cycles. It also assures the embryo development by establishing a better connection between the mother and the placenta. This potentially results in providing multiple benefits to the embryo at multiple levels.

Freezing methods

There can be an excess number of good quality embryos after the transfer of a single embryo, or sometimes, two embryos. These embryos can be successfully frozen to use them at a later date. There are two different techniques of freezing used for embryo storage:

  1. Slow-freezing, generally used for day 2 and 3 embryos.
  2. Ultra rapid technique of freezing known as Vitrification, used for day 4, 5 and 6 embryos.

Embryos are kept in reserve in liquid nitrogen at a temperature of -196° C irrespective of the method of freezing used.

Slow-freezing method

In this freezing process, the embryos are first placed in various cryoprotective solutions with an objective to withdraw water from them. This allows the embryos to shrink. They are then filled with straws and kept in the freezing unit at a temperature lowered to below –135° C, over four hours as per the controlled program. The straws are next reserved in a cryotank filled with liquid nitrogen at a temperature of -196° C.


In this freezing process, the embryos are obtained from morulae and blastocysts and are conserved by vitrification.

The blastocyst is first prepared by making a minute hole in the zona (shell around the embryo) with a laser. This protects the embryo cells from cryoinjury that might happen during the process of rapid-freezing.

The blastocyst is next dipped into a sticky and adhesive solution that contains high concentrations of cryoprotictant. It is next placed on a unique open pulled straw and directly transferred into a solution of liquid nitrogen. The whole freezing process takes hardly a few minutes.

Oocyte (egg) vitrification is also done to help women undergoing chemotherapy/radiation for the treatment of cancer. It is also done for young women who want to give birth to their own child, but yet have not found a partner.

Slow-freezing method Vs. Newer vitrification process

After the embryos are cryopreserved, they are shifted from the incubator where they were developing in a temperature and atmosphere similar to the reproductive tract of a woman. The embryos are transferred into a channel that contains alcohol product, like glycerol, and sucrose.

This cryoprotectant channel is important as it replaces the water present within the cells of the embryo. The channel is introduced into the embryo cells in different short timed stages in accordance with exact formulas, which have been formulated and determined as per the research works of over several years.

During the process of slow-freezing, the embryos are kept preserved in the different cryoprotectant concentration channels for 5 to 10 minutes on each of it. This creates a slow diffusion of the cryoprotectant solution into the embryo cells. The embryos are next placed separately into a straw-like device or vessel. They are next introduced into a cryopreservation freezing machine that reduces their temperature into two stages in the course of two hours. This slow decrease of temperature gradually increases the concentration of the cryoprotectant media. Thus, it results in the gradual increase in the quantity of cryoprotectant solution entering the embryo cells. After two hours, the embryos are removed and kept in another labeled holder and plunged promptly into a storage tank containing liquid nitrogen.

During the process of vitrification, the embryos are kept at considerably higher concentrations of cryoprotectant channel for the very short duration of time. This results in a faster movement of water drained out of the embryo. Once the water is drained out of the cells, high levels of concentrated cryoprotectant quickly enters the cells. The embryos are then shifted by the rapid drops of cryoprotectant and preserved into another labeled vessel measured in microdroplets. It is then directly dipped into liquid nitrogen. They are placed into a labeled holder and transferred into the liquid nitrogen storage tanks.

The slow-freezing method of embryo preservation has been applied in IVF treatment for the past twenty-five years, whereas, vitrification is the predominant freezing process everywhere now.

Various studies and researches performed on women with vigorous stimulation of their ovaries. Such women are considered to be at more risk of ovarian hyperstimulation syndrome (OHSS) due to forceful stimulation. IVF experts have hence started to skip direct or fresh embryo transfer and freeze all of the embryos instead for later use. This remarkably reduces the risks of severe or complicated OHSS.

However, in a few cases, IVF experts decide to move ahead with a fresh embryo transfer, despite there being the risk of OHSS. In both the cases, the couples have a large number of embryos and the best of the lot is selected for transfer in each case. There is no difference in the pregnancy success rates between fresh or frozen embryos. Freezing embryos give the opportunity of future pregnancy to the couple. It also gives hope to childless couples to have babies with frozen embryos.

Hence, it can be concluded that cryopreservation does not decrease the scopes or possibilities of pregnancy by itself. The number of quality embryos collected out of a fertilization is the determining factor of multiple successful pregnancies.