Embryo transfer

Embryo transfer refers to a step in the process of in vitro fertilization (IVF) whereby one or several embryos are placed into the uterus of the female with the intent to establish a pregnancy.

Fresh versus frozen
Embryos can be either "fresh" from fertilized egg cells of the same menstrual cycle, or "frozen", that is they have been generated in a preceding cyle, cryopreserved, and are thawed just prior to the transfer.

Uterine preparation
In the human, the uterine lining (endometrium) needs to be appropriately prepared so that the embryo(s) can implant. In a natural or stimulated cycle, the embryo transfer takes place in the luteal phase at a time where the lining is appropriately developed in relation to the status of the embryo. In a cycle where a "frozen" embryo is transferred, the recipient woman could be given first estrogen preparations (about 2 weeks), then a combination of estrogen and progesterone so that the lining becomes receptive for the embryo. The time of receptivity is the implantation window.

Timing
In stimulated cycles in human IVF, embryos are typically transferred 3 days after fertilization and may then be at the eight-cell stage, or they are transferred 2 to 3 days later when they have reached the blastocyst stage. Embryos who reach the day 3 cell stage can be tested for chromosal or specific genetic defects prior to possible transfer by preimplantation genetic diagnosis (PGD).

Procedure
The procedure of embryo transfer is performed by a physician, often with the aid of ultrasound to allow for precise placement. The catheter loaded with one or more embryos is advanced through the cervix into the uterine cavity and the embryo(s) are released into the cavity. Anesthesia is generally not required.

Embryo number
A major issue is how many embryos should be transferred. Placement of multiple embryos carries the risk of multiple pregnancy. In the past, physicans have often placed too many embryos in the hope to establish a pregnancy. However, the rise in multiple pregnancies has led to a reassessment of this approach. Professional societies and in many countries, the legislature, have issued guidelines or laws to curtail a practice of placing too many embryos in an attempt to reduce multiple pregnancies.

e-SET
The technique of selecting only one embryo to transfer to the woman is called elective-Single Embryo Transfer (e-SET). It eliminates the risk of multiple pregnancies, compared with e.g. Double Embryo Transfer (DET).

Follow-up
After embryo transfer patients are kept on estrogen and progesterone medication; pregnancy testing is done typically two weeks after the transfer.

Third-party reproduction
It is not necessary that the embryo transfer be performed on the female who provided the eggs. Thus another female whose uterus is appropriately prepared can receive the embryo and carry the pregnancy. This situation may apply for a woman who has eggs but no uterus and wants to have a biologic baby; she would require the help of a gestational carrier to carry the pregnancy. Also, a woman who has no eggs but a uterus may resort to egg donor IVF, in which case another woman would provide eggs for fertilization and the resulting embryos are placed into the uterus of the patient. Third party reproduction is controversial and regulated in many countries.