Preparation for a frozen embryo transfer
A frozen embryo transfer involves the use of thawed embryos that were frozen after an IVF cycle. The success of a FET cycle depends on the quality of the embryos upon freezing, the quality of the endometrium at the time of transfer and if the patient has any underlying diseases or anatomical anomalies.
At present, just about all assisted reproduction clinics worldwide vitrify embryos so in 2015, when we talk about a frozen embryo transfer we are almost always talking about a “devtrification cycle” although that term has not become as yet “layman’s language” as a frozen embryo transfer has.
Vitrification is a flash freezing method, which turns embryos after dehydration into a consistency of glass. This technique is a great improvement over slow freezing which was the only method used up to ten years ago. Ninety percent of embryos that undergo vitrification and devitrification survive.
There are two basic ways to prepare for a FET: one is using the natural cycle of the patient and which entails doing the cycle with her own hormones to prepare the endometrium and the other way is a substituted cycle using exogenous hormones prescribed and initiated on day+2 of the cycle which help the endometrium attain the correct thickness for embryo transfer.
There are advantages and disadvantages to each type of cycle. If the patient has regular cycles it is more physiological to prepare the patient with a natural cycle. The advantage is that there is much less medication to take. The great disadvantage of a natural cycle is that the day of the transfer is not predictable. The patient needs to ovulate the month that she wants to transfer and some months even the most fertile women do not ovulate. If this happens, the cycle is cancelled and the following month monitoring of the cycle is repeated to make sure a follicle grows and that the patient ovulates.
The advantage of a substituted cycle is that the transfer date is more flexible and within limits, the cycle can be accommodated to the patient’s needs. The disadvantage is that there is more medication.
There are conflicting opinions as to if it is better to undergo a natural or a substituted cycle. Some studies say the pregnancy rate is higher with natural cycles but the truth is, it is best to adapt the cycle to each patient taking into consideration previous cycles she has undergone and in tis way try and predict which type of cycle will give her the greatest chance to achieve pregnancy.