Fresh or frozen embryos: What do these terms mean?

Among the techniques we can perform in an IVF laboratory is the freezing of embryos and gametes (eggs and sperm). This is generally referred to as cryopreservation and is usually done using a method called vitrification. The gametes are frozen in liquid nitrogen at very low temperatures. Dr. Antonio Requena Miranda, a gynecologist specializing in Human Reproduction and Medical Director of the IVI-RMA Global Group, explains this process in this article, answering frequently asked questions.

Is freezing harmful to embryos?

With new freezing techniques (vitrification), in a very high percentage (over 90% according to centers) the embryo does not suffer during the freezing/thawing (devitrification) process, and we should have an embryo with a perfect capacity to implant and develop into a baby. In a small percentage, some damage to the embryo's structures may occur, causing it to become blocked after thawing and, therefore, preventing transfer. It's also important to know that pregnancy rates are currently similar for fresh and frozen embryos.

Can freezing cause any damage to the development of the fetus and the baby?

No, and there are already a large number of children born using this technique and there has never been a higher incidence of problems during pregnancy or in the child (for example, malformations) due to the use of these techniques.

Is there a time limit for embryos to be frozen?

There is no time limit except when the patient turns 50 in private fertility clinics and the age limit established by public clinics for treatment, depending on the region. It's important to know that once frozen, the embryo is not affected by the time it remains in this state.

Fresh embryos versus frozen embryos. Which is better?

We are talking about fresh embryos, which are those resulting from an In Vitro Fertilization cycle. These embryos can be transferred directly into the uterus or frozen for transfer in a later cycle. 

We should think that it's always best to be as minimally invasive as possible with embryos. Therefore, it seems logical to prefer a fresh embryo to a frozen one. But if we consider, as we've said, that the survival rate of embryos after freezing/thawing is very high, that it doesn't cause any problems for the embryo/fetus/baby, and that its potential to result in pregnancy is the same as with a fresh embryo, then it's no longer so clear which is better. Furthermore, we must consider that being able to freeze embryos allows us to perform diagnostic procedures, avoid complications for the mother, or store surplus embryos. 

In what cases would freezing embryos be indicated?

• Whenever we have more embryos than we are going to transfer, which should be in most patients a single embryo per transfer. 
• When we believe the patient's endometrium (the uterine lining where the embryo implants) is not in optimal condition after an IVF cycle, we will have a higher success rate if we freeze all the embryos and transfer them in a subsequent cycle with a healthy endometrium for implantation. 
• When there is a risk that the patient may develop ovarian hyperstimulation syndrome (OHSS). This syndrome worsens if the patient becomes pregnant. Therefore, it is best to freeze all the embryos, wait for the patient to have her period (which eliminates the risk of OHSS), and then transfer them in a subsequent cycle. 
• When we need to perform diagnostic techniques on embryos whose results take more than 5-6 days, which is the time the embryo can remain in an incubator before transfer. For example, when we perform an embryo biopsy for chromosomal or genetic testing. 
• In other cases where we think it is better not to do a fresh transfer.

So, do embryos always have to be frozen?

There is some controversy surrounding this, and some fertility clinics believe it's always best to freeze embryos after an IVF cycle and transfer them later into perfectly prepared endometrium. Other clinics believe that in some cases, a fresh embryo can be transferred during the IVF cycle, and if embryos remain, they can be frozen. It's important to discuss all of these issues with your doctor. Listen to their advice, but also express your doubts or preferences.

Article reviewed by Carlos Dosouto