What is Embryo Freezing/ Sperm Freezing/ Cryopreservation?

It is the procedure where sperms, eggs or embryos are preserved by cooling to low sub zero temperatures such as -196 degrees C. At such low temperature, any biological activity including biochemical reactions are effectively stopped.

Advantages / Use of cryopreservation


This procedure gives patients the option of having excess embryos frozen, to be used in future cycles. During IVF treatment, a maximum of three embryos can be transferred to the uterus in any one cycle. As modern methods of stimulation often result in more than three eggs being collected during a cycle, there may be excess embryos available following treatment.

  • Reducing the risk of multiple pregnancy by transferring fewer embryos in the fresh cycle and cryopreservation of supernumerary embryos.
  • Increasing cumulative pregnancy rate by using cryopreserved embryos in thaw cycles.
  • Since cryopreservation cycles do not require ovarian stimulation and ovum pick up, the treatment is significantly cheaper.
  • In patients at a risk of developing Ovarian HyperStimulation Syndrome (OHSS), a complication which develops when a large number of eggs are produced, all the embryos are cryopreserved to reduce the risk and severity of OHSS.
  • Incase of poor endometrial development and receptivity, as can be deduced using ultrasound scanning, all embryos will be cryopreserved. These embryos can then be transferred later after appropriate treatment.


  • When the husband is not available at the time of infertility treatment [staying abroad or travelling elsewhere].
  • When the husband is worried that he may not be able to produce a semen sample on the required day due to anxiety/stress.
  • When the sperm count fluctuates very much.
  • In men suffering from malignancies requiring chemotherapy or radiation which will affect sperm production.

Steps of Cryopreservation:

Only embryos of suitable quality, that are likely to survive the freeze-thaw process, are selected for freezing. The main principle of cryopreservation, for embryos, is to reduce damage caused by intracellular ice formation. Cells therefore, should undergo dehydration i.e. loose water. This is partially achieved by the addition of cryoprotectants which due to their higher molecular weight and osmolarity make cells to loose water.

Two basic techniques have been developed:

  • Controlled slow-rate cooling protocol in which lesser concentration of cryoprotectants is used and cooling is performed at a very slow-rate to avoid damage to the embryos.
  • An alternative method is a ultra-rapid protocol called Vitrification in which a very high concentration of cryoprotectants is used and embryos are frozen directly in liquid Nitrogen.  The chance of an embryo surviving a freeze and thaw is approximately 60 per cent.

Frozen embryos may be kept in storage for a maximum period of five years routinely. In some specific circumstances these may be stored for ten years or occasionally more.

  • Embryos are loaded with cryoprotectant into straws labeled with Patient's ID, Patient's name and number of embryos frozen .
  • Straws are sealed and transferred to a programmable biological freezer which is used to achieve a controlled slow rate of cooling.
  • During cooling, cells dehydrate and as the temperature is reduced more ice forms and water is removed gradually from the cells.
  • Slow cooling is continued to approx -35 degrees C, at which point embryos are rapidly cooled by plunging them in liquid nitrogen.
  • Embryos are kept in storage tanks of liquid nitrogen until thawing is performed. 

Steps In A Frozen Thaw Embryo Transfer Cycle

Embryos that have been frozen can be transferred at a later stage in 2 ways.

Natural Cycle :

This is possible in women who ovulate regularly. The cycle is monitored with sonography to determine the exact day of ovulation. Embryos are transferred 2 or 3 days after ovulation. No drugs are needed in this.

Artificial Cycle

This is helpful in women who do not ovulate regularly or when embryo transfer is to be programmed. The patient is given tablets from the 2nd day of periods for endometrial development. When the endometrial thickness becomes > 8mm, progesterone is added and embryo transfer carried out 2 or 3 days later.



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