Ever wonder about the differences between a fresh embryo transfer and a frozen embryo transfer in an IVF cycle? This month, Jim Meriano, our Chief Embryologist and Director of TRIO’s lab answers your scientific questions; and Dr. Robert Casper, our Scientific Director and a fertility specialist, answers your medical questions.

Ask an Embryologist

Jim Meriano, B.Sc., M.Sc., ELD, Director Embryology Lab at TRIO, answers your technical questions about Fresh and Frozen Embryo Transfer


1. What is the difference between a fresh and a frozen cycle?

When you do your retrieval you will likely have a number of eggs collected. These will become embryos after they are fertilized in our lab. Canadian guidelines allow one embryo per cycle to be transferred. If you have additional embryos, they will be frozen in case you wish to transfer them later. When embryos are frozen and then thawed for transfer, this is referred to as a frozen cycle. A fresh cycle occurs 5 days after your retrieval: when the egg is fertilized, and the embryo grows to the blastocyst or day 5 stage, which is optimal for transfer.

2. Do embryos have the same possibility of success whether they are fresh or frozen?

About 15 years ago, when embryos were first frozen with the old method, there was a significant difference in embryo quality. Back then, pregnancy rates were much higher with fresh cycles. Today, however, the technique for freezing embryos is extremely reliable. So much so that there is no difference in embryo quality between fresh and frozen. So, the short answer is yes, fresh and frozen embryos have the same possibility of success.

3. What is vitrification and will it affect the embryo?

Vitrification is the technical term used for freezing eggs and embryos. The technique is a fast and effective freezing method. Because the technique does not allow ice crystals to form, it is safer for the embryo. Therefore the answer is no, vitrification does not affect the embryo. 

4. How long can frozen embryos be stored before their quality diminishes?

Vitrification has been around clinically for about 15 years, and embryos that have been frozen over that time period and then thawed have been viable. At TRIO, all embryos that have been frozen are always checked for their viability once they are thawed and before they are implanted.

5. Is there a way to determine the viability of frozen embryos before implantation?

First the frozen embryo is thawed. Then it is examined under a high-powered microscope in our embryology lab for viability in order to determine whether or not it is suitable for transfer.

6. How are the embryos stored?

Frozen embryos are stored safely and securely in tanks of liquid nitrogen. These remain in the TRIO lab under carefully controlled temperatures.


Ask a Fertility Doctor:

Dr. Robert F. Casper MD, FRCSC, REI, Scientific Director and Founding Partner of TRIO, answers your medical questions about Fresh and Frozen Embryo Transfer


1. Why would I choose to freeze my embryos?

There are a number of significant reasons a patient may choose a frozen cycle over a fresh one. Here are some of the key reasons, in brief:

  • One of the most important reasons we may recommend a frozen embryo cycle over a fresh one is if the patient is at elevated risk for OHSS, or Ovarian Hyper Stimulation Syndrome, a rare but serious side effect that may happen when medication stimulates too many eggs in one cycle. For some patients, choosing a frozen embryo transfer cycle can reduce this risk by removing high levels of the hormones (hCG) that could act as triggers during the transfer portion of the cycle, and opting to freeze the embryos for transfer in a subsequent cycle.


  • If a woman’s estrogen levels are very high during a cycle and she produces, for example, 10 follicles or more, her progesterone will be higher as well. This may affect the successful implantation of the embryo. However, if we freeze the embryos and transfer them in a subsequent cycle when the patient’s hormone levels are more like they would be in a natural cycle, the chances of that embryo implanting in her uterus are much better.


  • Recent studies and stats indicate that the pregnancy rate is about 5 percent higher with a frozen embryo transfer because it mimics a physiologic cycle more closely and therefore a higher chance of implantation.


  • Often, extra embryos are created during a cycle. We can freeze these additional embryos for subsequent transfer at the day 5 or blastocyst stage, where the embryos are more mature and have a higher chance of implantation. A blastocyst will also take advantage of implantation at its peak.

2. What is the difference in the treatment cycle for a woman planning a fresh embryo transfer and a frozen embryo transfer?

A fresh treatment cycle is a stimulated cycle, where multiple follicles are created. When the eggs are retrieved and the embryos created, the transfer must take place shortly after the retrieval, and according to the patient’s hormone levels. A frozen embryo transfer occurs in a non-stimulated cycle, and we can use a more natural estrogen and progesterone combination to prepare for the implantation. The frozen transfer may also be less stressful, as only one or two ultrasounds are required, so the patient does not have to come to the clinic as often. Finally, a frozen embryo transfer offers the patient more flexibility, as she can schedule the day she wants her transfer and book her own TRIO doctor to do the procedure.

3. Does it make sense to freeze all embryos and wait for implantation?

Some clinics in the United States have recently eliminated fresh transfers in favour of frozen  transfers primarily because frozen cycles offer a slight increase in pregnancy rate, a reduced risk of OHSS and more flexibility for the patient.  

4. Will embryo freezing affect the health of the baby?

Because frozen embryo transfers have been around for many years, we have a lot of data to back up their success. In fact, recent data suggests there is an increased risk of low birth weight infants in fresh transfers because of the increased hormones (estrogen and progesterone) in the mother due to the stimulation cycle she has just completed to create the oocytes. In frozen embryo cycles, it now appears that because there is less elevated estrogen, there is greater endometrial receptivity and the weight of the babies at birth is more normal.