Embryo Transfer

Embryo transfer at a glance

  • Embryo transfer occurs during in vitro fertilization (IVF) when a fertility specialist places an embryo created in the laboratory into the woman’s uterus for implantation and pregnancy.
  • Embryos are typically transferred at either three days or five days after fertilization.
  • An elective single embryo transfer (eSET) is sometimes recommended to reduce the risk of a multiple pregnancy (twins or more), due to increased health risks to the mother and child.
  • The procedure for embryo transfer is the same whether embryos are from a fresh cycle or frozen cycle (embryos previously frozen and stored) prior to transfer.

What is an embryo transfer?

An embryo transfer refers to the part of the IVF procedure in which a physician uses an ultrasound to guide a catheter containing the IVF-produced embryo(s) that places the embryo(s) directly into the uterus. The embryo transfer process only takes a few minutes, does not involve anesthesia and only requires a short recovery period.

Embryos develop through multiple stages, often described by the number of days since the egg was retrieved from the woman’s follicle. For example, day one, day two, day three, day four, day five or day six, all occur after day zero, which refers to the date the egg was retrieved and fertilized.

Embryologists will grade embryos and the type of grading depends on the stage of the embryo. For cleavage stage embryos, typically on day three, the number of cells and a grade (A – D) will be assigned. For blastocysts, there will be a number and two letters assigned. The number refers to the amount of expansion of the fluid (the “cyst”) in the blastocyst. The two letters (A – D) that follow refer to the inner cell mass (destined to become the baby) and the trophectoderm (destined to become the placenta), respectively.

Cells from an embryo can also be tested for genetic anomalies prior to an embryo transfer. Scientists have a choice of two genetic tests for embryos. In preimplantation genetic diagnosis (PGD), an embryologist removes a group of cells to test for a specific genetic abnormality, such as cystic fibrosis. Preimplantation genetic screening (PGS) tests for the proper makeup in all chromosome pairs, as missing or additional chromosomes lead to disorders and diseases. An example of such a disorder is Down syndrome, in which there is an extra chromosome in pair number 21.

Blastocyst transfer

A blastocyst transfer involves growing embryos in a lab for five days before transferring them into the uterus. When the embryo has reached the blastocyst stage (day five), it is more fully developed with multiple cells.

At this point the embryo resembles the stage an embryo would naturally be when it enters a uterus for implantation, which increases the chances of achieving a successful pregnancy. However, not all embryos are able to develop to the blastocyst stage.

Studies show that blastocyst transfers result in higher implantation and pregnancy rates as compared with cleavage stage embryos. Blastocyst transfers may be of particular benefit for patients who develop many good quality embryos, who have failed to achieve a pregnancy with a day three transfer in the past, or who have poor quality embryos at day three.

Cleavage stage embryo transfer

A cleavage stage embryo transfer refers to embryos that are transferred at an earlier stage of development when they have fewer cells, typically six to eight, and occurs on day two or three after fertilization.

Cleavage refers to the division of the cells in an early developing embryo. Cleavage stage embryo transfer is a good option for patients who have fewer good quality embryos. Also, transfer on day three is less risky than allowing the embryos to go to day five.

Gamete intrafallopian transfer (GIFT)

Gamete intrafallopian transfer (GIFT) is a rare technique in which a specialist places the mixed sperm and eggs immediately into the fallopian tube using ultrasound guidance, rather than developing the embryo in the laboratory. The only justification for GIFT is for couples who have strong Catholic faith, as this is the assisted reproductive technology allowed by the Vatican.

GIFT is performed by laparoscopy, in which the doctor inserts a thin tube with camera and surgical instruments through the belly button. In order to perform GIFT, a patient must have at least one normal fallopian tube, as documented by an X-ray (hysterosalpingogram test), and the absence of scar tissue on the outside of the tube, as documented by laparoscopy.

Fresh vs. frozen embryo transfers

Transfers can involve either fresh or frozen embryos. A fresh embryo transfer means that the eggs are removed, fertilized to become embryos, cultured for several days in the lab, and then transferred into the uterus. This occurs during one menstrual cycle, and the embryos are never frozen.

A frozen embryo transfer means that good quality embryos resulting from an egg retrieval cycle are frozen for later use. Frozen embryos can be used during the next menstrual cycle or months (or years) in the future.

How many embryos are transferred?

It is important to determine how many embryos will be transferred. This subject is highly debated. According to the American Society for Reproductive Medicine (ARSM), the goal of assisted reproductive medicine is to achieve a single gestation, or a single birth through treatments.

Elective single embryo transfer (eSET) refers to only transferring one embryo in a cycle. The eSET option is usually recommended when patients have a high chance of becoming pregnant on their first embryo transfer cycle – for example, a woman who is under the age of 35 and has good embryo quality and availability.

In other instances, two or more embryos are transferred to increase the chances of achieving a successful pregnancy. However, the transfer of more than one embryo can result in a multiple pregnancy (meaning twins, triplets or more). This could cause serious health risks for both the mother and baby.

Risks include low birth rate, preterm labor or delivery and fetal loss. A high-order multiple pregnancy, in which three or more embryos are implanted, is an undesirable outcome of assisted reproductive technologies. 

The decision as to number of embryos to transfer should be made by the patient and physician and should take into account several factors, including:

  • Quality and quantity of embryos available for transfer.
  • Developmental stage of the embryos at transfer.
  • Age and medical history of the patient, including previous IVF treatment.

While there are minimal risks associated with embryo transfers, there is a risk that none of the embryos will survive in the lab, that embryos may be damaged in the lab, and that embryos will not be suitable for a transfer.