Thawing the Mysteries of FET
Medical Contribution By Dr. Anitha Nair
About 40 percent of patients who undergo IVF have an additional embryo (or embryos) which they choose to cryopreserve (freeze) to use for another attempt, should their first cycle be unsuccessful, or to continue to build their family at a later date. This process is called a frozen embryo transfer (FET). But what goes into a FET cycle, and what are the chances that those frozen embryos will become a baby?
“For patients with a good prognosis and who are 35 years old or younger, we [at Shady Grove Fertility] can offer 50 percent success rates,” says Dr. Anitha Nair of Shady Grove Fertility’s Washington, DC office. “In turn, this means that the success rates for FET cycles are nearly as high as IVF cycles using fresh embryos.”
So How Do You “Freeze” an Embryo?
Once a couple has gone through an embryo transfer, any extra embryos that were not transferred and have made it to the blastocycst stage (day 5) are frozen by a process called vitrification. This process is done by placing the embryo into a solution and then rapidly freezing it in liquid nitrogen. This rapid freeze increases the cooling rate and prevents the formation of ice crystals (which may cause breakage later). The most important part about this new technique is benefit of being able to thaw and transfer back fewer embryos compared to the “slow freeze” process which is traditionally used.
“[Prior to vitrification] in order to optimize a patient's chances at pregnancy, we would transfer back one additional thawed embryo than we would transfer in a fresh IVF cycle,” explained Dr. Nair. “But with vitrification, we're more often transferring the same numbers of embryos, fresh or frozen with nearly the same pregnancy success rate.”
The FET Cycle
When the patient decides it's time to conceive using her frozen embryos, a pre-medication ultrasound is performed to ensure there are no cysts or other structural problems. Then, medications will be administered to optimize the lining of her uterus for implantation. Estrogen and progesterone injections are given every third day for up to two to three weeks, and the uterine lining is checked by post-medication ultrasound.
Dr. Nair explains that, although there are still medication protocols to follow, when a patient comes in for a FET cycle, they find the process far easier and less stressful and far less medication than undergoing a “fresh” IVF cycle.
“The whole FET process is less intense, and requires far fewer office visits,” explains Dr. Nair. “The patient will use less medication since all we're doing is stimulating her uterine lining to prepare for implantation. No ‘super’ ovulation fertility medications are called for with an FET. Plus, there's usually only need for one ultrasound monitoring to check her lining, as opposed to the frequent monitoring done with ‘fresh’ cycles.”
After the two-plus week preparation, it's time to thaw the frozen embryos and transfer them to the hopeful mother. Thawing takes very little time, just about an hour, so 'transfer day' is eventful.
“We're conservative and start thawing only the number we intend to transfer,” Dr. Nair explains. “If those initial embryos don't come through the process in good shape, we'll thaw more if necessary. Generally, almost nine out of 10 embryos survive the thawing process.”
Dr. Nair says that one of the interesting benefits of FET for many patients is that it's a little like freezing time.
“Let's say a patient goes in for her IVF at the age of 33 and freezes some of the resulting embryos,” she said. “When she returns in a few years for an FET for her second baby, her ovaries and eggs will have aged and her fertility will have statistically decreased somewhat, but her frozen embryos are just as good cellularly as they were when she was younger. It's like a head-start in the getting-pregnant process for older women.”