Sishu Fertility Clinic

What is embryo freezing? Fresh Vs Frozen Embryo Transfer

What is embryo freezing?

Embryo freezing (Cryopreservation) is preserving embryos by freezing them at sub-zero temperatures using a vitrification technique. Freezing keeps the embryo safe and can be used for pregnancy later in life. 

How is embryo freezing done?

The embryos are cooled using the vitrification technique and stored in liquid nitrogen at around -196 degrees Celsius (-320 degrees Fahrenheit).

Let us understand step by step:

  • IVF Procedure: Initially, eggs are retrieved from ovaries and fertilized with sperm in a laboratory to create embryos.
  • Embryo Development: These embryos are cultured in a controlled environment for several days until they reach a suitable stage of development, the blastocyst stage.
  • Freezing: Embryo freezing is done using the vitrification technique to prevent the formation of ice crystals that can damage the cells.
  • Storage: The frozen embryos are stored in liquid nitrogen. 

Fresh vs Frozen Embryo Transfer

Fresh Embryo Transfer (Fresh ET)

Fresh embryo transfer is a procedure in which Freshly collected embryos are transferred directly into a woman’s uterus. Immediate embryo transfer is done after 3 to 5 days after egg retrieval.

Process Of Unfrozen Embryo

  • Embryos are created in a lab outside the woman’s body.
  • Healthy embryos are transferred directly into the uterus.

Advantages Of Non-frozen Embryo

  • Shorter cycle: The process takes about a week.
  • Immediate transfer: Transfer occurs right after retrieval.


  • Risk of Ovarian Hyperstimulation Syndrome (OHSS): Due to medication.

Limited time for genetic testing: Immediate transfer may limit the window for genetic analysis.

Frozen Embryo Transfer (FET)

Frozen embryo transfer is a procedure in which previously cryopreserved (frozen) embryos are thawed and transferred into a woman’s uterus. Embryos are frozen for future use.

Process of Frozen Embryo Transfer

  • Embryos are created in a lab and then frozen.
  • Implantation can occur weeks, months, or even years later.

Advantages of frozen Embryo Transfer

  • Preimplantation Genetic Testing (PGT): Provides options for genetic testing before transfer.
  • Reduced risk of OHSS: Lower risk compared to fresh transfer.
  • Flexibility in timing: Allows for scheduling convenience.


  • Longer overall process: Takes more time compared to fresh transfer.
  • Requires embryo thawing: Embryos must be thawed before transfer.

Type of Transfer




Fresh Embryo Transfer (FET)

Embryos created in lab, transferred directly to uterus after 3-5 days

Shorter cycle, immediate transfer

Risk of OHSS, limited time for genetic testing

Frozen Embryo Transfer (FET)

Embryos created in lab, frozen, thawed before transfer to uterus

PGT possible, reduced OHSS risk, flexible timing

Longer process, requires embryo thawing

Leave a Reply

Your email address will not be published. Required fields are marked *