There are 2 methods of Cryopreservation of your Embryos and Eggs: Vitrification and Slow Freezing. Understanding the pros and cons of these processes will help you decide which is best suited for your condition.

Why we prefer Vitrification over Slow Freezing for cryopreservation of your Embryos and Eggs?

Dr. Saiprasad Gundeti, Senior Embryologist, Malpani Infertility Clinic Pvt. Ltd.

Since most IVF programmes superovulate patients to grow many eggs, there are often many .

At Malpani Infertility Clinic, since we do not transfer more than 3 Embryos , many are left with "spare" or supernumerary . These can be cryopreserved.

In addition, there are conditions under which we may recommend that all embryos be frozen and that no embryo replacement be performed during your IVF treatment cycle. For example, if you are at high risk for OHSS ( ovarian hyperstimulation syndrome ); or if your endometrial lining is poor.

The frozen are stored in liquid nitrogen. These frozen can be stored for as long as is needed - even for many years. When they are in liquid nitrogen, at a temperature of -196 deg. C, they are in a state of suspended animation, and all metabolic activity at this low temperature stops.

These stored can then be used later for you, so you she can have another embryo transfer cycle done without having to go through superovulation and egg collection all over again.

For women with irregular menstrual cycles, frozen can also be done in a "simulated natural cycle", in which the endometrium is primed to maximize its receptivity to the by using exogenous estrogens and progesterone. Since pregnancy rates with good-quality frozen-thawed are as good as with fresh , we encourage all our to freeze and store their supernumerary , rather than discard them.

However, since it is worthwhile only good quality , the option of is a "bonus" which is available to only about 30% of all IVF . The risk of defects is not increased as a result of

There are 2 techniques of cryopreservation of your embryos:

  • Slow programmable freezing
  • Vitrification

Slow programmable freezing

Controlled-rate and slow freezing, also called slow programmable freezing (SPF), is a set of well established techniques used for cryopreservation of your embryos.

Lethal intracellular freezing can be avoided if cooling is slow enough to permit sufficient water to leave the cell during progressive freezing of the extracellular fluid.

A typical cooling rate around 1oC/minute is appropriate for human embryos after treatment with cryoprotectants such as glycerol or dimethyl sulphoxide.

In this techniques of controlled-rate freezing the embryos are slowly cooled in cryoprotectant fluid ("anti-freeze" solution) from body temperature down to -196oC and at this temperature they are stored in containers of liquid nitrogen called canisters.

Programmable Freezer for controlled Slow freezing of embryos

Your embryos are stored within special indelibly labeled plastic vials, or straws, that are sealed prior to freezing. Once frozen, they are placed inside coloured tubes Called visitubes and stored in numbered canisters within the liquid nitrogen cryocans. The site and label details are stored in a Register, to avoid confusion and misidentification of cryopreserved embryos.

When it comes time to thaw the embryos, all available identifiers of the stored specimen must match and be confirmed before thawing commences.

The embryos are thawed out at room temperature, which takes about 25 minutes. However, the most critical element of the thaw procedure is not the timing but the careful dilution of the cryoprotectant fluid to return the embryo to its favored culture medium. This permits resumed growth and development in vitro.

Once this is done, the embryo is assessed for cryodamage to determine if it is suitable for transfer.

While this technique has been used for many years to cryopreserve human embryos, it is not commonly used when it comes to cryopreserving eggs. This is because eggs are much larger and have a higher intracellular water content. As a result of this, they are more fragile when subjected to the stresses of slow freezing; and do not survive well after thawing.

Even for embryos , the survival rate after thawing ( even in expert hands) is only about 50%.

Embryos and Egg verification

The embryo in the picture is a Post - Thaw embryo which was frozen using programmable slow freezing technique. Out of the 4 cells, you can see that 1 cell is damaged( it's turned dark), due to ice crystal formation in that particular cell, which has killed that cell.

Cons of Slow programmable freezing of embryos

  • It involves slowly cooling down the embryo until it finally freezes. This makes it a very time consuming procedure.
  • The major disadvantage of this approach is that it causes ice crystals to form inside the embryo's cells (blastomeres), damaging them and thus reducing viability.
  • Poor pregnancy rate due to poor survival of embryos.
  • Cannot freeze Eggs with this technique, due to uncertain survival rate.


Vitrification is a newer technique which uses ultra-rapid cooling, together with a much higher concentration of cryoprotectants. Vitrification is used for cryopreservation of eggs and embryos , and does not cause damage due to ice crystal formation. Vitrification requires the addition of cryoprotectants prior to cooling. The cryoprotectants act like antifreeze: they lower the freezing temperature. They also increase the viscosity. Instead of crystallizing, the syrupy solution turns into an amorphous ice i.e., it vitrifies.

Two conditions usually required to allow vitrification are an increase in the viscosity and a depression of the freezing temperature.

At Malpani Infertility Clinic, we have been using vitrification for a number of years to cryopreserve both embryos and eggs.

When it comes to embryo cryopreservation, we freeze all stage of embryos i.e. Pronuclei stage (day 1 embryos) , multicellar stage (Day 2, Day 3 embryos), Blastocyst Stage (Day 5, Day 6).

Vitrification has improved our potential to successfully bank human eggs. We run a very successful Egg bank at Malpani Infertility Clinic.

This embryo is a fresh 4-Cell Grade A embryo. We vitirified this.

This an embryo was thawed after being cryopreserved using vitrification technique.

The embryo looks exactly the same as it was before freezing. None of the cells have been damaged and they are all intact.

Pros of Vitrificaiton technique :

  • This ultrarapid process is so fast that it literally allows no time for intracellular ice to form. As a result, vitrification avoids trauma to the embryo.
  • Vitrified embryos have a better than 95% freeze-thaw survival rate, and a pregnancy generating potential that is comparable to fresh embryos.
  • High survival rate enables us to preserve eggs safely and reliably.

Vitrification is superior to Slow Freezing of Embryos.

Slow Programmable freezing


  • It causes ice crystals to form within embryo's cells (blastomeres).
  • Unsatisfactory survival rate. Only 50 % of embryo survive.
  • All embryos do not survive the freeze-thaw, and those that do survive have less than half the likelihood of generating a pregnancy as do fresh embryos.
  • Very time consuming. Embryos are slowly cooled using a programmable freezer before they are actually frozen. It takes approx. 2 hours to freeze embryos using this technique.
  • No ice crystals are formed inside cells as freezing occurs ultrarapidly.
  • Optimal survival rate is achieved. About 95% of embryos and eggs survive.
  • Excellent survival of embryos and pregnancy generating potential that is comparable to fresh embryos.
  • Doesn't take much time with this technique. As the embryos are cooled ultrarapidly. It takes approx. 30 min to vitrify 10 embryos.
Authored by : Dr Aniruddha Malpani, MD and reviewed by Dr Anjali Malpani.

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