A cycle of in vitro fertilisation (IVF) can be completed using frozen embryos as well as fresh ones. This can be useful in several circumstances. If you have spare embryos after a successful IVF cycle, you can have a frozen embryo transfer (FET) cycle done a couple of years later to avoid having to stimulate your ovaries again. Frozen embryo transfer is also used for women who are using donated embryos, or for women who need cancer treatment at a young age.
After a successful IVF cycle, which may have included intracytoplasmic sperm injection (ICSI), one or two of the best embryos or blastocysts that developed are transferred into the uterus. If you have several other high-quality embryos, these are frozen quickly so that they can be stored for use in the future.
Freezing seems safe for the embryo. Evidence shows that children born as a result of a frozen embryo transfer are no more likely to have birth defects than children born as a result of IVF with fresh embryos.
Embryos are frozen using a vitrification technique. This involves rapid freezing to prevent any ice crystals forming in the cells of the embryo. The faster the freezing process, the more likely it is that a frozen embryo transfer will result in a successful pregnancy.
Frozen embryos are stored in a special solution in a sterile vial inside a container of liquid nitrogen, which has a temperature of minus 196 degrees Celsius. Labelling is done very carefully and the embryos can only be thawed with the written consent of both parents. Embryos are known to survive for many years once they are frozen, and embryos are currently allowed to be kept for 10 years.
When you have an IVF cycle and use your fresh embryos, the uterus is ready to accept transfer of embryos. If you have a frozen embryo transfer cycle, you may have to have the transfer coordinated with your natural menstrual cycle (if you have one). This can be difficult because it requires constant monitoring to determine when you ovulate so that frozen embryo transfer can be done on the correct day.
Alternatively, your uterus can be prepared using hormone treatments. This involves:
Frozen embryo transfer then takes place a set number of days after progesterone treatment begins. The uterus is only ‘ready’ for a short time, and if this window of opportunity is missed, the treatment cycle needs to start again.
Timing varies depending on whether the frozen embryo transfer is being done with 2–3 day embryos or 5–6 day embryos (frozen blastocyst embryo transfer). Blastocysts are usually transferred on day 5 of progesterone treatment.
A frozen embryo transfer for 2–3 day embryos and blastocysts is done in exactly the same way as a fresh embryo transfer. Only some of the embryos are thawed to start with, a process that takes about an hour. These are checked and if two high-quality embryos are available, these are transferred into your uterus.
If you are using embryos frozen immediately after fertilisation, when they are still at the pronuclear stage, the procedure is slightly different. As these embryos are literally only one cell at the point of freezing, there is no way to tell how good quality they are.
They must be thawed and cultured in the laboratory for at least 2–3 days until they reach a stage where they can be assessed and, if necessary, tested using pre-implantation screening techniques. This can make coordinating the process somewhat trickier as it means making sure the embryos and your uterus are ready on the same day.
Probably the most common motivation for frozen embryo transfer is to use spare embryos generated in a previous IVF cycle. This not only reduces the cost, if you are having private infertility treatment, but it also means you do not have to go through another round of treatment to stimulate your ovaries. The more times you do this, the greater your risk of developing ovarian hyperstimulation syndrome (OHSS).
Other situations in which a frozen embryo transfer may be used include:
Although the chances of pregnancy are lower with frozen embryo transfer, the data is open to interpretation. If the best embryos are chosen for transfer in the initial IVF cycle, the frozen embryos may be not quite as high quality, so a drop in success rate in the next cycle would not be a surprise. When high quality embryos are compared, there seems to be no difference in the likelihood of getting pregnant whether you have fresh or frozen embryo transfer.