Frozen Embryo Transfer

Embryo Transfer – Frozen or Fresh

An embryo transfer is the process of placing an embryo in a woman’s uterus to establish a pregnancy. This process may use frozen or fresh embryos, with each process having a specific set of considerations.

Embryo Transfer – Overview

Embryos can be created in the laboratory by fertilizing an egg with a sperm cell, a process known as in vitro fertilization. The embryo can be implanted immediately into a woman’s uterus, but it can also be frozen while the embryo is still alive. This procedure is known medically as cryopreservation. A frozen embryo requires a fertility specialist to thaw the embryo according to a specific procedure known as the frozen embryo transfer cycle. Both frozen and fresh embryos require the woman’s uterus to be prepared for implantation with a series of hormone supplements.

The primary challenge with cryopreservation is to freeze the embryo without killing it. Salts build up in the cells of the embryo when it freezes, which will normally reach toxic levels. A fertility specialist must place the embryo in a solution that contains some type of chemical to prevent this from happening, usually glycerol. The embryo is then frozen in liquid nitrogen with specialized machinery. Embryos stored in this manner can last up to 13 years, and clinical pregnancies have been created with embryos that have been stored for 9 years. The thawing process generally requires no more than 45 minutes, and it is performed just before the frozen embryo transfer.


Frozen embryos generally have the same rate of developmental abnormalities and birth defects as fresh embryos. The average birth weight of babies born from frozen embryos is slightly higher than that of babies born from fresh embryos. The primary advantage of frozen embryos is that it allows you to use them at some point in the future without going through the egg collection procedure again, which is typically expensive and inconvenient. Frozen embryos also allow the implantation procedure to be canceled while still preserving the embryos. This often occurs when the woman has a bad reaction to the drugs required for implanting the embryo. Freezing the embryos also allows you to donate them to other women or for medical research.

The number of embryos to use is an important issue in embryo transfer. More embryos increase the chances of a viable pregnancy, but they also increases the chances of a multiple pregnancy. Medical organizations in most countries place specific restrictions on the number of embryos that can be transferred in order to reduce the number of multiple pregnancies produced by this procedure. A physician generally must transfer a larger number of fresh embryos since a failed pregnancy means the egg collection procedure must be repeated. Frozen embryo transfers typically involve a smaller number of embryos since more embryos can be transferred with relative ease in the event of a failed pregnancy.

Risks of Frozen Embryo Transfer

The primary difference in risk between frozen and fresh embryos is the chances of a multiple pregnancy. Multiple pregnancies generally increase the danger to both the mother and babies. The transfer of a single embryo to a woman’s uterus is called an elective-single embryo transfer, which has a 3.5 percent chance of producing twins. A similar procedure called an elective single blastocyst transfer is used for embryos at the blastocyst stage. An eSBT has a 2 percent chance of producing twins. A double embryo transfer has a 38 percent chance of producing twins, while a double blastocyst transfer has a 25 percent chance of producing twins.