In IVF treatment, the embryos obtained are sometimes frozen for use in a later month. When the transfer month arrives, the focus is no longer on growing eggs but on preparing the inner lining of the uterus — the endometrium — where the embryo will implant. Although the question our patients most often ask, "How many millimetres should the uterine lining be?", is an important one, it is not right to reduce the entire preparation to a single measurement. The thickness and appearance of the endometrium, the condition of the ovaries, the timing of the hormones, and the developmental day on which the embryo was frozen are all evaluated together.

At our clinic we use two main approaches for frozen embryo transfer: the natural or modified natural cycle, in which we follow the patient's own ovulation, and the artificial — also known as hormone-supported — cycle, in which we prepare the uterine lining with medication. The shared aim of both methods is to identify precisely the window in which the endometrium is receptive to the embryo. We choose the method not by asking "which one is more successful for everyone?" but rather "which one is safer, more practicable and more controllable for this particular patient?"

Why is timing important in frozen embryo transfer?

Endometrium adet döngüsü boyunca değişir. Östrojen etkisiyle kalınlaşır; yumurtlamadan sonra salgılanan progesteronun etkisiyle embriyonun tutunmasına uygun bir yapıya dönüşür. Bu dönüşümün başladığı gün ile embriyonun gelişim yaşı uyumlu olmalıdır. Örneğin blastosist aşamasında dondurulmuş bir embriyonun transfer zamanı, daha erken gelişim gününde dondurulmuş bir embriyodan farklı planlanır.

Bu nedenle transfer günü yalnızca takvimdeki adet gününe bakılarak belirlenmez. Doğal siklusta LH yükselişi, yumurtlama veya çatlatma iğnesinin zamanı; hormonlu siklusta ise progesterona başlanan saat ve toplam progesteron maruziyeti dikkate alınır. Küçük görünen bir zaman farkı bile planın değişmesini gerektirebilir. İlaç saatlerinin, kontrol günlerinin ve kliniğin verdiği transfer tarihinin aynen uygulanmasını istememizin nedeni budur.

Endometrial preparation in natural and modified natural cycles

In a natural cycle, the endometrium is prepared by the oestrogen secreted from the patient's own developing follicle and, after ovulation, by the progesterone produced by the corpus luteum. This method is generally considered for patients who have regular periods and ovulate spontaneously. Our clinic's follow-up form specifically asks about the menstrual pattern over the past six months; regular cycles of roughly 24–35 days are regarded as more suitable for natural-cycle monitoring.

The first assessment is usually carried out on the second day of the period, or on the third if necessary. Using transvaginal ultrasound, we check the uterine lining, both ovaries and whether there is any cyst that could affect the cycle. Depending on the ultrasound findings, hormone tests such as E2, LH and progesterone may be requested. If nothing on the initial assessment stands in the way, a repeat check is usually scheduled around day 9 or 10 of the cycle, according to its length. In patients with shorter cycles, this check may be carried out earlier.

Once the growing follicle reaches around 15–16 mm, monitoring becomes more frequent. Follicle diameter and endometrial thickness are measured by ultrasound, and where necessary ovulation is followed closely with E2, LH and progesterone results. In a modified natural cycle, a trigger injection containing hCG may be given at the physician's discretion to make the timing of ovulation more predictable. In a true natural cycle, the spontaneous LH rise and ovulation are taken as the reference. If the follicle has ruptured earlier than expected, the progesterone result is also evaluated and the transfer schedule is recalculated.

The advantage of the natural cycle is that the uterine lining is prepared by the patient's own hormones and, in most patients, requires less hormone medication from outside. On the other hand, closer ultrasound and blood monitoring may be needed so that the day of ovulation is not missed. Check-ups may fall on a weekend, and if the cycle progresses differently than expected the plan may have to be postponed. Being "more natural", therefore, does not automatically mean it is easier for every patient.

The artificial cycle — preparation with hormones

In an artificial cycle, instead of waiting for ovulation, the endometrium is prepared with oestrogen and progesterone given from outside. This approach may be preferred in patients whose periods are irregular, whose ovulation cannot be predicted, or who need the transfer date to be planned in a more controlled way. For patients monitored in another city or country, the fact that it allows a set schedule can also be a considerable convenience. Even so, the decision is not made on logistical grounds alone; the medical history, the response of previous cycles and the possible risks of the medications are always taken into account.

At the beginning of the transfer month, contact is made with our clinic, usually within the first days of the period. The initial ultrasound checks whether the ovaries are quiescent, whether any cyst is present, and how the endometrium looks at baseline. If a cyst or an unexpected follicle is seen, E2, LH and progesterone results may be requested. If the findings are suitable, oestrogen treatment is started and around days 10–11 the thickness and pattern of the endometrium are reassessed. Where necessary, a further check is carried out on days 14–15.

In our forms, cases where the endometrium remains below 7 mm are evaluated separately. However, the transfer decision is not made on a single millimetre threshold alone. Whether the measurement was taken in the correct plane, the ultrasound appearance of the uterine lining, whether there is fluid or another problem within the uterus, and the patient's results in previous treatments are all considered together. When the endometrium remains thin, the oestrogen dose or the route of administration may be changed, the duration of treatment may be extended, or the transfer may be postponed to another month.

Once the endometrium is judged suitable, progesterone is started. The transfer day is determined by matching the time progesterone was begun with the developmental day of the embryo. From this stage onwards, it is particularly important that no dose is skipped and that the times of the medications are not changed. Although the transfer day and the pregnancy test day are shown on our treatment charts, this chart is not a sample flow but an instruction specific to that patient. Another patient's drug dose or transfer day must never be taken as an example.

In a hormone-prepared cycle, a follicle may sometimes develop unexpectedly. In such a case, the medications being used are not stopped on one's own initiative. By evaluating the follicle size and the hormone results, monitoring may be continued in a manner similar to a natural cycle, or the plan may be rearranged. This decision is made by our clinic.

Which method do we consider for whom?

No single preparation method is best for all patients. In a patient who has regular periods and ovulates reliably, a natural or modified natural cycle may be appropriate. Where there is irregular menstruation, an ovulation disorder, a polycystic ovarian appearance, or a need to plan the date in a more controlled way, the hormone-supported cycle may come to the fore. The way the endometrium responded in previous transfers, repeated cycle cancellations, intrauterine findings, other medications being taken and the patient's general health also influence the choice.

Whether the patient lives outside the city is also important for how workable the plan is. In remote follow-up, however, the basic principle does not change: ultrasound images and reports must reach us on the same day, by the time our team has specified. It is not only the thickness written in the report that is assessed, but the endometrial measurement image as well. Together with any hormone results deemed necessary, the next check-up, any change of medication and the transfer date are communicated by our clinic.

Why is the FET follow-up form so detailed?

Our blank FET follow-up form brings together on a single chart the menstrual pattern, whether the cycle is natural or artificial, previous treatments, endometrial thickness, follicle measurements in the right and left ovaries, E2, LH and progesterone results, and the medications used. It also records critical stages such as the developmental day of the embryo, the thawing time, the transfer date and the pregnancy test.

The purpose of this level of detail is not to leave the patient alone with a complicated chart, but to ensure communication and traceability within the team. Every new ultrasound or hormone result is compared with the previous values. In this way it can be determined more safely whether the treatment is progressing as planned, whether the next check needs to be brought forward, and how the transfer window should be calculated.

The aim of each check-up is to determine the next step at the right time and to safely reduce the risk of unnecessary medication use or of transferring too early.

Points we ask our patients to pay attention to

• Report the day your period starts in the way our clinic has described; ask whether spotting should or should not be counted as day one.
• Along with the ultrasound report, please also send the endometrial measurement image if possible.
• Have the requested blood tests done on the same day and send the results by the deadline you have been given.
• Do not change the timing of the trigger injection, progesterone or any other medication on your own decision.
• If you miss a dose, or if you experience unexpected bleeding, severe pain or any other complaint, please contact our team.
• Do not use another patient's form, medication list or a schedule found on the internet for your own treatment.

Frequently Asked Questions

Is a thick uterine lining on its own enough for transfer?

No. Endometrial thickness is an important monitoring criterion, but on its own it does not mean the lining is ready. We also assess the ultrasound pattern, whether there is fluid or a space-occupying lesion within the uterus, the hormone values and the timing at which progesterone was started. How the measurement changes from one check to the next matters at least as much as any single result. For this reason, a value considered suitable in one patient may not lead to the same decision in another.

In what situations can the cycle be postponed?

A hormone-producing cyst seen at the outset, unexpected ovulation, insufficient development of the endometrium, fluid observed within the uterus, or any situation in which the timing of the transfer cannot be established safely may lead to the cycle being replanned. In a natural cycle, being unable to track the LH rise or ovulation may also require postponement. Postponement does not mean the embryo has been lost; the embryo remains preserved in its frozen state. The aim is to prepare a more controlled cycle rather than to carry out the transfer on an unsuitable day.

Until when are the medications used after the transfer?

This period varies according to the preparation method and the individual plan. In a hormone-supported cycle, oestrogen and progesterone support may need to continue for some time even after the pregnancy test. In a natural cycle, additional progesterone support may be given even though the corpus luteum produces hormones. Whether the pregnancy test is negative or positive, the medications must not be stopped without the clinic's approval. The test date, the plan for reducing or stopping the doses, and the timing of the first pregnancy ultrasound are communicated separately by the team.

Our clinic's approach: the same goal, a personalized path

Successful preparation for a frozen embryo transfer does not simply mean saying "the uterine lining has thickened". Our aim is to ensure that the endometrium reaches a suitable appearance and development, that the hormones take effect in the correct order, and that the developmental day of the embryo is matched with the receptive phase of the uterine lining. Natural, modified natural and hormone-supported cycles are different paths used to reach this same goal.

At our clinic we choose the method according to the patient's menstrual pattern, ovulatory characteristics, ultrasound and hormone results, previous attempts, medical history and follow-up circumstances. The reason we use standard forms throughout the cycle is not to give everyone the same treatment, but to carry out individualised decisions in an orderly and safe way.

Answering the patient's questions at every stage of the plan, comparing the results within the same team and communicating any possible changes promptly are also an inseparable part of our clinical approach. In this way the process remains medically traceable and, for the patient, clear, simple and easy to understand.

If you are planning a frozen embryo transfer, you are welcome to contact us for an assessment, bringing the details of your existing embryos together with your previous treatment records. The endometrial preparation method and follow-up schedule best suited to you will be drawn up after your examination.

This content is intended for general information purposes only. For starting or stopping any medication, or for changing a dose, please follow the advice of the physician managing your personal treatment plan.