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What is a natural FET cycle?

6 fertility expert(s) answered this question

Answer from: Raúl Olivares, MD

Gynaecologist, Medical Director & Owner Barcelona IVF

In cases where the patients have their natural cycle, there is the possibility of working with the natural cycle. That means that instead of giving oestrogens in pills, which are usually done in cycles of hormone replacement therapy, we are just going to leave the ovaries working as they would do in a natural cycle.  In these cases, patients get in contact with us on day 1 of their cycle, and then we schedule the first scan on day 8-9 of the cycle, just to measure the dominant follicle.  We may need more scans because we follow how this follicle grows and whenever the follicle reaches 17-18 millimetres, we trigger the ovulation with any hCG injections, and it is usually done, for example, in IVF or IUI.

What we want in this cycle or what we try to do, is to prepare the endometrium using the patient’s own hormones, rather than giving some external hormones. Though the pregnancy rates of hormone replacement therapies and natural cycles seem to be very similar, according to the big studies,  in cases where we are using hormones, the outcomes are not as we’re expecting. It makes sense to work with the natural cycle. We are going to change the way the endometrium is going to be prepared, and the patient is going to take fewer drugs.  Who knows, we may be improving the quality of the embryo and increasing the implantation rates.

Answer from: Tomas Frgala, PhD

Gynaecologist, Head Physician at UNICA Clinic - Brno

What needs an embryo that has been successfully cultivated and previously cryopreserved, it needs for the transfer. It needs a bed that is nicely made in the uterine cavity represented by the endometrium, the uterine lining that is nicely grown. Two hormones are necessary for that. In the first half of the cycle, it’s estrogen, in the second half of the cycle also progesterone. We can either supply these hormones via pills or vaginal suppositories in a regulated or supported cycle with the hormonal replacement therapy – that is one way how to get prepared for the transfer or we can let the endometrium grow naturally in the native cycle – just observing the finding with ultrasound (usually performed at first around day 10 or 11) and based on the size of the dominant follicle that produces the hormones at this phase, at the initial phase (mainly estrogen) and based on the size of the endometrium, we can try and estimate the moment (the date) of the ovulation and then, take it from there. In a truly natural frozen embryo transfer cycle – no hormones are added – just estrogen eventually LH, progesterone are measured via blood tests, ultrasounds are performed and as I have mentioned, the ovulation time is estimated. There are however many modifications to this cycle and one that gives us perhaps a little more accuracy and assurance is a modification in which we actually help time the ovulation with a single shot of ovidrel or pregnyl (mainly HCG). 36 hours later, the ovulation should take place and sometimes after that ovulation, we also perform a so-called luteal phase support where we add a little bit of extra progesterone but mainly the natural frozen embryo transfer cycle is one where we let the endometrium go naturally and just step in around the half of the cycle – just to prepare the best possible conditions for the embryo.

Answer from: Alpesh Doshi

Embryologist, Consultant Embryologist and Co founder at IVF London

Natural frozen embryo transfers are much more monitored compared to medicated frozen embryo transfers and it works very much in line with the patient’s own natural menstrual cycle so, for example, they would take no hormones. Typically we would ask the woman or the patient to come at around day 10 of her cycle – literally natural menstrual cycle and we would look at, via ultrasound, we would look at the thickness of the endometrium or the uterus and we would also look at what we only give one injection called the trigger injection to release that egg – we’re not interested in that egg of course but just so that the process of the endometrium building up can start. It’s more involved, there’s more scans involved but at the same time no hormones involved and the natural hormones in the body are assisting in building up the lining of the uterus. It must be said that a natural cycle of frozen embryo transfers are very rarely done these days because there is better control in medicated frozen embryo transfers compared to natural cycles. There is a larger number of natural cycles that get canceled simply because we may have missed that window of transferring the embryo back at the right time.

Answer from: Harry Karpouzis, MD, MRCOG, DIUE

Gynaecologist, Founder & Scientific Director Pelargos IVF Medical Group

A natural frozen cycle is a cycle that happens without the medication to prepare the endometrium, this means a natural cycle without external estrogen. We don’t use it often. We prefer to use medication. When is it used? What we usually do is the initial scan on the second day of the period and we follow up the follicles in this cycle till the time they are ready for ovulation. Depending on how many days of the cycle a woman has, the follicle can be ready in about 10-16 or even more days. We usually prefer to see a follicle that reaches about 20-21 mm of size before it is ready for ovulation.

From the time that the follicle is more than 18 mm, we usually advise doing an ovulation test using urine or even measure the LH levels in the blood so that we know we have the correct time coming. Then, after that, we do the embryo transfer. Sometimes we prefer to give a trigger injection so that we can schedule better the time of the embryo transfer. The time of the embryo transfer depends on how many embryos we have and on which day these embryos had been frozen. So if they are day 3 embryos, the embryo transfer happens 3 days later; if they are day 5 embryos, it happens 5 days later.

Answer from: Anna Voskuilen, MD

Gynaecologist, Specialist in Obstetrics, Gynecology and Reproductive Medicine Reproclinic S.L.

First, let me explain what happens in a normal cycle. At the beginning of a normal cycle, the follicles are available to grow but we don’t have hormones to make all of them grow. That’s the reason why we ovulate just one egg, and during this growth of these follicles that are going to cause maturation of the eggs, the follicle will also produce oestrogens. The oestrogen then makes the endometrium lining grow.

After the ovulation, what happens? In our ovaries we have the Corpus Luteum that is like a scar, it stays there after the ovulation and produces progesterone so we will have estrogens but a lot of progesterone. This progesterone is going to make changes in the endometrial lining so that in case there’s an embryo there, it can allow the embryo to implant on it. When we don’t have an embryo, the hormone levels are going to decrease, and we will have menstruation.

So what do we do with a natural cycle to prepare for an embryo transfer? We try to use all these natural processes that we have in our body. So after the beginning of our cycles on day 8 or 9, we will start doing some controls with blood tests and scans to see how the follicle and the endometrial lining are growing.

After that, we normally do what’s called Modified Natural Cycle which means we trigger the ovulation with medication so that we know exactly when you’re ovulating because we need to have the endometrium totally synchronized with the embryo that we’re going to transfer. How do we do that? We do it with the medication so we know exactly when you’re ovulating. We are using the hormones that are created naturally and which make the endometrium lining grow and make it perfect for implantation, and, also, we use corpus luteum, the scar, that is in the ovary after the ovulation, that produces the progesterone in a natural way. Actually, it is like mimicking a natural conception when doing the embryo transfer. The disadvantage with this is that it is not as flexible as a medicated cycle so we cannot control the cycle phases. With a medicated cycle we will need to continue the medication during some weeks of pregnancy but with a natural cycle, we don’t need to continue hormonal treatments.

Answer from: Oksana Babula, MD

Gynaecologist, Fertility Specialist EGV Clinic

Natural frozen embryo transfer – this is my favorite. I love everything natural so natural means that you don’t take any hormones. What do we need for natural embryo transfer? We need you to have ovulation. If you are not ovulating, if you don’t have ovulation, you are not going to have the transfer. We need ovulation; we can stimulate ovulation then it’s medication again. But it’s only for five days, for example, we can use Clostilbegyt for five days to have you ovulate. There are some pluses that you have to get less medication, that it’s natural and also the results are a little bit better if you’re comparing with the stimulated cycle. Minuses are that we have to catch ovulation so we will need more frequent ultrasound checkups. Our clinic is not working on Sundays and on holidays so if your ovulation is on a holiday or during Saturday or Sunday and we are not sure what was the day of your ovulation then we will have to cancel the embryo transfer.

Ovulation – how it works? You are coming to the doctor around the 9-10 day of your cycle and the doctor is checking your dominant follicle. When the dominant follicle size is this miracle number 21-22 millimetre, it usually ovulates. Sometimes we help ovulation with the HCG injection. Sometimes, we will let things go absolutely naturally so we just check your ovary and make sure that ovulation has happened. Sometimes you have to do an ultrasound every day. Then, when gynecologists see this fresh corpus luteum that is a sign that ovulation happened, so we see that ovulation has happened so we calculate depending on the embryo whether it’s day three embryo or day five embryo so the ovulation day is day zero. Then, we calculate day five and on day five we do the transfer.

If it’s a frozen embryo transfer, then the embryo is already frozen and waiting for you to be ready for the transfer. The thawing is on the day of the transfer so when the endometrium is perfect. Usually, the embryos are thawed in the morning and in the afternoon we do the embryo transfer so-called frozen embryo transfer. To tell you the truth, the results don’t differ from the fresh embryo transfer so no difference here.

We can use progesterone or it can be done absolutely naturally without any hormones depending on the quality of your endometrium. If the endometrium is thick enough, if it looks good, we say the endometrium looks as if it is smiling: three lines of the endometrium. Then if the endometrium is smiling at me, I say everything looks perfect, it’s smiling, everything is going to be perfect. Of course, sometimes if we are working with an advanced age lady who is 45+, 50+, then we give also some medication to improve circulation like aspirin, low-dose aspirin 100-150. Some patients want to be absolutely medication-free so absolutely natural which is wonderful as long as it works, whatever you prefer. I always would choose the natural one, not the medicated one. The results are wonderful. Actually, there are fewer complications, there are publications that say that the fewer medications we use for embryo transfer the healthier the baby will be, fewer chances of having any kind of endocrinological or any other problems later in life both for the lady and for the baby.

About this question:

How does the natural frozen embryo transfer cycle work?

A natural FET (Frozen Embryo Transfer) cycle, where no medications are taken, IVF cycles are monitored by checking the hormone levels using blood tests and by doing an ultrasound to monitor the thickness of the endometrium. How does it work?

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