Frozen Embryo Transfer process
Bay IVF - Advanced Reproductive Care                                Palo Alto  650-322-0500


Frozen Embryo Transfer Process

Reproductive treatments may include embryo freezing (cryopreservation, vitrification) and the storage of frozen embryos in liquid nitrogen. At a later date, the stored embryos can be thawed and transferred into the uterus. Embryo cryopreservation, storage, and the Frozen Embryo Transfer process are done on-site at Bay IVF.

Vitrification uses extremely high cooling rates. Water is transformed directly from the liquid phase to a glassy vitrified state. The definition of vitrification is the solidification of a solution at a low temperature, not by ice crystallization but by extreme elevation in viscosity during cooling.

In the past, with "slow" freezing methods, about half of the embryos did not successfully survive the cryopreservation-thawing process. With vitrification, the embryo survival rate is close to 100%. At Bay IVF, we use the vitrification method exclusively for embryo cryopreservation.

If you have embryos intended for cryopreservation, they will be cultured to the blastocyst stage (day 5-7 embryos) and then vitrified.

What is Frozen Embryo Transfer?
Conceiving with your cryopreserved embryos will require the Frozen Embryo Transfer process. Embryo cryopreservation, thawing, and Frozen Embryo Transfer are very safe. Hundreds of thousands of babies have been born worldwide from cryopreserved embryos. There is no indication of any increase in the rate of birth defects, and the implantation rate of the thawed embryos is equal to the "fresh", unfrozen embryo implantation rate.

The Frozen Embryo Transfer procedure step by step:

  1. Preparation of endometrial lining
  2. Thawing and culture of cryopreserved embryos
  3. Assisted embryo hatching
  4. Embryo transfer
  5. Implantation


  1. Preparation of Endometrial Lining

    There are several different forms of endometrial lining stimulation protocols, each with many modifications. Your treatment is always individualized.

    Below is an example of endometrial stimulation protocols used at Bay IVF. Your cryopreserved embryo transfer cycle may take less or more time to complete.

Endometrial Stimulation Example


Endometrial Stimulation Example


The treatment begins with taking oral contraceptives. They are started within the first four days of the beginning of a menstrual cycle. A few days after the last pill, you will have a menstrual period, and the endometrial stimulation will begin at that time.

Oral and injectable medications are used to prepare your endometrial lining for the transfer of frozen-thawed embryos. The endometrial lining preparation is straightforward, and you should not expect any side effects.

  1. Thawing and Culture of Cryopreserved Embryos

    You will need to decide and communicate to us whether you would like to thaw and transfer one or two embryos. Please let us know if you need help with this decision.

    Selecting one or two embryos will eliminate the risk of a high-order multiple pregnancy (triplets or more). With this approach, most pregnancies from cryopreserved embryos are single-baby pregnancies.

  2. Assisted Hatching

    Assisted hatching is a laboratory procedure used to create a "weak spot" in the eggshell of an embryo. Since embryo freezing commonly hardens the embryo's eggshell, assisted embryo hatching is always a part of the Frozen Embryo Transfer treatment.

    This picture shows an embryo after assisted embryo hatching with an opening breaching the eggshell at the 12 o’clock position.


Assisted Hatching


  1. Embryo Transfer

    Just prior to your embryo transfer, the embryos are placed into the tip of a thin embryo transfer catheter. The catheter is then passed through the cervical canal to within 15 mm of the top of the uterine cavity, and the embryos are gently released.

  2. Implantation

    What happens after Frozen Embryo Transfer?
    After the embryo transfer, the endometrial lining gently holds the embryo(s) at the top of the uterus. There is no restriction on your physical activity.

    A blood pregnancy test is done ten days after the embryo transfer. If the pregnancy test is positive, an ultrasound examination is scheduled two weeks later to visualize the implantation site and look for a heartbeat within the embryo. Once a heartbeat is seen, there is a 95% probability that the pregnancy will continue to live birth.


Pregnancy Ultrasound


This ultrasound picture shows a six-week pregnancy. The pregnancy sac is 25 mm in diameter. The baby inside the sac is only 13 mm long. It is already possible to distinguish the baby’s head and body and to see the cardiac activity.



    There is no increased risk of birth defects in pregnancies from cryopreserved embryos compared with conceptions conceived through intercourse or using "fresh" embryos.

    Supplementation of estrogen and progesterone must continue until the placenta produces enough estrogen and progesterone to sustain the pregnancy. You will be closely monitored for 6 to 8 weeks as this transition takes place. Once all medications are discontinued, you will be referred to your OB doctor for the remainder of your obstetrical care.

    At this point, your pregnancy becomes indistinguishable from conception through intercourse, and your obstetrical care should be no different than if you conceived without any treatment.


Please use the following links for additional information about Frozen Embryo Transfer and reproductive treatments at Bay IVF.



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