Process and methods of freezing
Process and methods of freezing

Oocyte Freezing

Oocytes that are retrieved from the patient can be frozen and stored irrelevant of the maturation stage.  Unfortunately, this method does not yield high pregnancy rates.  Lately however, vitrification has been a more promising method for freezing eggs.  This method can be applied to women who have to undergo chemotherapy or who do not have a partner and want to delay motherhood.

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Embryo freezing

If there are excess good quality embryos after embryo transfer, these can be frozen and stored.  Despite the fact that not all embryos will survive the freeze-thaw procedure, pregnancy rates have approached fresh embryo transfers.

Embryo freezing has become an established method and has been practiced for nearly 20 years.  Particularly in situations where a high number of oocytes have been retrieved, freezing after embryo formation has provided a second chance for couples whose first attempt did not result in pregnancy.  Preparation of the patient for a thawed embryo transfer is medically simpler and less intrusive.  In some cases, patients come back to transfer the stored embryos to conceive a second child.

Embryos can be frozen at any point during their development.  One can see fertilized eggs on the first day, and then the zygote begins dividing to become an embryo. On the 5th day the embryo becomes a blastocyst, a stage which shows better results after the freeze/thaw procedure.

Briefly, embryo freezing is a procedure that adapts to the developmental stage of the embryo.  Embryos are placed in prepared media designed to protect embryos from extreme low temperatures.  An instrument, controlled by a computer, slowly reduces the temperature of the solution until it reaches –80C  Embryos are then labeled and stored individually in tanks filled with Liquid Nitrogen which maintains a temperature of –196C. No biological activity can be observed at these temperatures and embryos can be stored until the patient is ready for them to be thawed. The freeze thaw procedure yields 20% loss of vitality and those embryos that survive have a subsequent 1/3 chance of implantation when compared with fresh embryos.  Technology, however, is continually improving survival rates

Technically frozen embryos can be stored indefinitely depending on the couple’s decision. Although conventionally embryos are stored for a maximum of 3-5 years, case reports show that embryos can be stored for longer periods of time.  By having couples renew their contract yearly, they can decide whether they want to thaw and transfer the embryos back to the uterus, destroy them or donate the embryos to research.

The Health Ministry in our country has extended the duration of embryo storage to 5 years.

Electronic records are made for all patients and only authorized people with a password can access.  These records have 5 backups made daily and stored in different places.  Two copies are stored in another building for safety.


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This is a method where embryos are frozen quickly to prevent ice crystal formation.  When the solution freezes, it solidifies to form a glass like appearance.

The goal of this procedure is to prevent ice crystal formation inside the cell while the temperature is reduced.  DMSO (DimethylSulfoxide) and Ethylene Glycol are the most commonly used cryoprotectants.

After the embryos are labeled, they are plunged directly into Liquid Nitrogen.

Currently vitrification has been becoming popular among IVF clinics as the method has shown similar or even better survival rates and pregnancy rates than the slow freezing process besides being less time consuming for the laboratory staff.

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Freezing tissue biopsied from testes

After a testicular biopsy is taken and sperm is found, the sample obtained can be frozen and stored in –196C.

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Sperm Freezing

In the event that the male has to have radiotherapy and/or chemotherapy that can lead to infertility. If husband cannot be present on the day of his partner’s eggs collection due to various reasons such as a business trip or psychological reasons, sperm can be frozen in advance and stored.  Unlike freezing embryos and oocytes, survival rates of sperm after freezing and thawing are high. Pregnancy rates following sperm freezing and thawing are close to those of fresh sperm.

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Freezing Ovarian Tissue

This is the method where prior to beginning chemotherapy, a biopsy is taken from the cortex of the ovary and frozen in –196C.  This method has received a lot of media attention however; there have not been any proven pregnancies as a result of this procedure although a few reports claiming pregnancies after the autotransplantation of the thawed ovarian tissue. All these pregnanies happened in women who had their own ovaries. Therefore the possibility of ovulating from the existing ovaries creating pregnancies cannot be excluded.

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