Frozen Embryos 101
By Karen Synesiou, CFO, Center for Surrogate Parenting, Inc.

How long do frozen embryos last?

It is generally believed that embryos frozen for 10 years can still produce a viable pregnancy and subsequent live birth. However there is one birth that occurred after the embryo was frozen for 13 years. ( The real issue is that most couples use their embryos within four to five years after creating them. Couples with frozen embryos want a pregnancy and either will use their embryos until either a pregnancy is achieved or until there are no remaining embryos for implantation. It is unusual to find a couple who creates frozen embryos and then waits 10 years before implanting them. Equally it is unusual to find a couple that has a child and resulting frozen embryos to wait 10 years to have another child. Most couples prefer children born closer in age to one another. It is for all these reasons that it is infrequent to find couples implanting frozen embryos more than five years after their creation.

What is a frozen embryo?

When embryos are created and not immediately implanted into a recipient uterus these embryos need to be cryopreserved/frozen or they will die within a few days.
Sometimes when embryos are created:
  • There are more embryos than can be implanted

  • The recipients lining is too thin causing the cycle to be cancelled or delayed

  • The recipient reacts to the fertility medication and is too ill to continue with the cycle

  • The recipient’s health is not good enough to continue with the cycle (for example she is about to undergo cancer treatment or an operation)

  • For other reasons the recipient decides not to continue with the transfer.

Many couples decide to freeze some or all of their embryos so they have the opportunity to use them at another time or to be used if this transfer does not result in a pregnancy.

Another benefit of freezing embryos is that this allows couples to have several IVF cycles from the same egg collection and therefore reduced the number of times that the ovaries have to be stimulated.

How are embryos frozen?

Every IVF clinic has its own protocol for freezing embryos. Embryos can be cryopreserved at various stages of development ranging from one day after fertilization to five or six days after fertilization.

The process of freezing embryos takes a few hours and involves bathing the embryos in a cryoprotectant solution to prevent ice crystals from forming inside the embryo and destroying it. The cryoprotectant draws moisture out of the embryo and thereby prevents ice crystals from forming between the cells and destroying the embryo. Each embryo is placed on a straw or vial and in a computerized freezer the temperature is lowered in various stages from room temperature to around minus 130 degrees, Fahrenheit. The straws or vials containing the embryos are then placed into the embryo storage tanks that are filled with liquid nitrogen.

There has been a move away from the 1980’s method of embryo freezing to the more widely used technology of vitrification.

According to Wikipedia: Vitrification is a process of converting a material into a glass-like amorphous solid that is free from any crystalline structure, either by the quick removal or addition of heat, or by mixing with an additive.
Simplified: vitrification is an ultra-rapid freezing process involving the addition of a cryoprotectant. The resulting embryos can then be rapidly cooled according to a specific protocol that avoids the formation of ice crystals that if allowed to develop will damage the embryos. It is believed that the vitrification process has allowed for better post-thaw embryo survival rates and higher live birth rates from frozen embryos. Most IVF labs have already switched to the vitrification technology.

Are frozen embryos transferred to the uterus?

No, the frozen embryos must be brought back to room temperature before being transferred to the uterus. Embryos are allowed to thaw naturally. The embryos must be washed to remove the cryoprotectant used during the freeze process. Typically embryos are "washed" in four separate solutions. The embryos are then pulled up into a catheter and are ready for implantation into the recipient's uterus.

Success Rate of Frozen embryos

It is not unusual for approximately 70% of frozen embryos to survive the thaw process. In part this is attributed to the "human factor." In other words, the more the embryos are touched or kept in a non-natural environment, the higher the chance of damage. Another view is that only the fittest of the embryos can survive the process and those that cannot survive the freeze and the thaw process probably had an inherent problem that caused them to deteriorate.

When asking your reproductive endocrinologist about his/her frozen success rates discuss the following:
  • The maternal age of the egg

  • The typical number of frozen embryos transferred

  • The percentage of embryo thaw cycles that resulted in a transfer. In other words how many cycles were cancelled due to lack of viable embryos to transfer?

  • The live birth rate per transfer procedure for frozen embryos

  • Ask for the above rates for surrogate pregnancies versus a couple's own pregnancy (this is important because surrogate mothers generally do not have a history of miscarriage and have proven fertility due to having their own children versus a couple who are doing IVF because they are unable to achieve a pregnancy or due to recurrent miscarriages.)

  • How many single embryo transfers have been undertaken and the statistics for resulting pregnancies

  • The rate of twins or multiple pregnancies

Cost of a frozen transfer

Some IVF clinics offer programs that, for the cost of a fresh transfer, will include the cost of a resulting frozen cycle should one be needed. A typical frozen embryos cycle costs around $3,000 for the IVF clinic and around $1,500 for medication needed to prepare the recipients uterus to accept the embryos.