Frozen embryo transfer: natural vs medicated cycle

More and more embryo transfers today are performed with frozen (vitrified) embryos, in a cycle separate from egg retrieval. The question that arises is how the endometrium will be prepared: by letting the woman’s natural cycle work, or by replacing the hormones with medication.

This choice is not only about the chance of pregnancy; it also appears to affect the safety of the pregnancy. That is why it is worth discussing carefully with your doctor.

What are the natural and the medicated FET cycles?

In the natural cycle FET, we monitor the woman’s spontaneous follicle growth and ovulation. The embryo is transferred in synchrony with natural ovulation; the corpus luteum that forms produces progesterone on its own. In a variant (modified natural cycle) ovulation is triggered with an injection.

In the medicated or programmed cycle, we give oestrogen and then progesterone to prepare the endometrium, without ovulation and without a corpus luteum. The advantage is that the transfer date is easy to schedule.

Which method yields more pregnancies?

In terms of the chance of pregnancy and live birth, the two approaches are broadly comparable. Neither has been clearly shown to be superior to the other for the birth rate.

For that reason, the choice rests less on effectiveness and more on other criteria: the regularity of your cycles, the convenience of scheduling and — increasingly — the safety of the pregnancy.

Why is the natural cycle said to be safer for the pregnancy?

Many studies and meta-analyses show that the medicated (programmed) cycle is associated with a higher risk of hypertensive disorders of pregnancy, especially pre-eclampsia, compared with the natural cycle. In several analyses this risk appears roughly doubled.

The most likely explanation is the absence of the corpus luteum in the medicated cycle. Besides progesterone, the corpus luteum produces vasoactive substances that seem to help the normal adaptation of the mother’s cardiovascular system in pregnancy.

Is progesterone needed in the natural cycle?

In the true natural cycle, the corpus luteum produces progesterone on its own, so the need for extra medicated support is theoretically smaller. In practice, many centres still give some luteal support anyway, since the data are not conclusive and the safety margin is high.

By contrast, in the medicated cycle progesterone is absolutely essential: without a corpus luteum, the pregnancy depends entirely on it being given from outside for several weeks.

Which woman suits each method?

The natural cycle suits women with regular cycles and normal ovulation. It requires more monitoring visits but offers the advantage of a lower risk of hypertensive disorders.

The medicated cycle is useful when cycles are irregular or ovulation is absent, when precise scheduling is needed, or in special endometrial situations. The choice is always individualized.

What should you discuss with your doctor?

Ask which method is recommended in your case and why. If you have regular cycles, ask whether a natural cycle is feasible, given the possible benefit for pregnancy safety.

Neither method is “wrong”; the decision simply needs to balance the convenience of scheduling against the data on hypertensive disorders, based on your own history.

Summary

The natural and the medicated frozen-embryo-transfer cycles give similar pregnancy rates. The crucial difference concerns pregnancy safety: the medicated (programmed) cycle is associated with a higher risk of hypertensive disorders and pre-eclampsia, most likely because of the absence of the corpus luteum. For women with regular cycles, the natural cycle is often the preferable option. The decision is always individualized, balancing safety and practicality.

Sources: meta-analyses of obstetric and perinatal outcomes after natural vs medicated-cycle frozen embryo transfer (Hum Reprod); ESHRE guidance on endometrial preparation for frozen embryo transfer; data on increased hypertensive disorders of pregnancy after programmed (medicated) cycles.

⚕ Medically reviewed by Prof. Stratis Kolibianakis · Thessaloniki Medical Association Reg. No. 16340

ARTICLE 2 · IVF