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Embryo Freezing and Frozen Embryo Transfer (FET) Explained
Twenty years ago, freezing an embryo meant accepting a lower chance of it working. That is no longer true. A change in freezing technique has been one of the quietest but most significant advances in fertility medicine, and it has reshaped how IVF cycles are planned. Today a frozen embryo transfer is at least as successful as a fresh one in most situations, and in several specific circumstances it is clearly safer. This guide explains how embryo freezing works, what an FET cycle involves, and why your clinic may recommend freezing everything rather than transferring straight away.
Why Freezing Changed
The old method, slow freezing, lowered the temperature gradually. The problem was ice. Ice crystals forming inside cells damaged them, and a meaningful proportion of embryos did not survive thawing.
Vitrification replaced it. The embryo is exposed to a protective solution and then cooled so rapidly that water has no time to crystallise; it passes straight into a glass-like solid state. Survival rates after thawing are now very high, and the embryo that comes out is, for practical purposes, the embryo that went in.
"Freezing is no longer a compromise. In many cycles it is the better plan, chosen deliberately rather than as a fallback."
Why Embryos Are Frozen
- Surplus good embryos from a fresh cycle. One stimulation and egg collection often produces more than one usable embryo. Freezing the rest means a second or third attempt without repeating the injections.
- Freeze-all to avoid ovarian hyperstimulation. Where a woman has responded strongly, transferring in that cycle raises OHSS risk considerably. Freezing everything and transferring later removes that risk almost entirely. See IVF injections and their side effects.
- A uterine lining that is not ready. High hormone levels during stimulation can make the endometrium less receptive. A later natural or prepared cycle gives a better lining.
- Progesterone rising too early during stimulation, which shifts the implantation window out of step with the embryo.
- Genetic testing of embryos, which requires time for results before transfer.
- Fertility preservation before chemotherapy or radiotherapy. Our oncology department raises this with younger patients before treatment starts.
- A medical or personal reason to delay, such as illness, surgery or travel.
- Accumulating embryos across cycles where ovarian reserve is low, so that a transfer happens only once there is a reasonable number banked. See the AMH test and ovarian reserve.
Fresh Versus Frozen Transfer
| Fresh transfer | Frozen transfer | |
|---|---|---|
| Timing | 3–5 days after egg collection, in the same cycle | In a later cycle, weeks or years afterwards |
| Hormone environment | High oestrogen from stimulation | Natural or carefully prepared |
| OHSS risk | Present, and increased by pregnancy | Effectively removed |
| Physical demand | Follows immediately on from collection | Body has recovered; no injections for stimulation |
| Scheduling | Fixed by the cycle | Flexible, easier to plan around work and travel |
| Success | Good | Equal or better in most groups, particularly strong responders |
Fresh transfer remains appropriate for many patients, particularly those with a normal response and a good lining. The point is that neither is automatically superior; the choice should be made on your response in that specific cycle.
How a Frozen Embryo Transfer Cycle Runs
An FET cycle is considerably lighter than a stimulation cycle. There is no egg collection, no sedation and no ovarian stimulation.
Preparing the lining: two approaches
- Natural cycle FET. Your own ovulation is tracked by ultrasound and blood tests, and the transfer is timed to it. Suitable for women with regular cycles, and uses minimal medication.
- Hormone-prepared (medicated) FET. Oestrogen tablets or patches build the lining, then progesterone is added to open the implantation window. This gives precise control over timing and is used where cycles are irregular.
The sequence
- Baseline scan at the start of the cycle.
- Lining preparation over roughly two weeks, monitored by ultrasound for endometrial thickness and pattern.
- Progesterone started once the lining is ready, which sets the transfer date.
- Thawing on the morning of transfer, with the embryologist confirming survival before you proceed.
- Transfer — a fine catheter passes through the cervix under ultrasound guidance. It takes a few minutes, needs no anaesthesia, and feels similar to a smear test.
- Luteal support with progesterone, continuing until the pregnancy test and usually into early pregnancy.
- Blood pregnancy test about ten to fourteen days later.
Common Questions About Storage
- How long can embryos be stored? Vitrified embryos do not deteriorate measurably with time in storage. Healthy babies have been born from embryos frozen for many years. Storage duration is governed by national regulations and your clinic's consent forms rather than by biology.
- Does freezing harm the embryo? Modern vitrification has high survival rates, and large studies have not shown an increase in birth defects in babies born from frozen embryos compared with fresh.
- What if an embryo does not survive thawing? A small proportion do not. If you have more than one stored, another can usually be thawed the same day, which is one reason clinics discuss how many to bank.
- What happens to embryos we do not use? You will be asked to record your wishes in writing in advance — continued storage, donation for research where permitted, or allowing them to perish. It is worth discussing this as a couple before you sign, not afterwards.
- Are there storage fees? Yes, usually annual. Ask for the figure in writing at the outset and check what happens if a payment is missed.
Frozen Transfer and Pregnancy Outcomes
Babies conceived from frozen embryos are, on average, slightly heavier at birth than those from fresh transfers, and the risk of low birth weight and preterm birth is somewhat lower. On the other side, medicated frozen cycles carry a slightly higher rate of hypertensive disorders in pregnancy, which is one reason natural-cycle FET is preferred where a woman ovulates regularly.
Whatever the route, early antenatal booking matters. Read what happens at the first antenatal visit and, where a medical condition already exists, how high-risk pregnancy care works. Delivery is supported by our obstetrics service and an on-site Level-III neonatal unit.
Discussing Freezing in Ambikapur
Whether to transfer fresh or freeze is a decision made during your cycle, based on how your ovaries respond and how the lining develops — so it is worth understanding both options before you start. Our IVF and fertility centre in Ambikapur discusses the plan at the outset and reviews it at each monitoring scan. Related reading includes what IVF, ICSI and IUI involve, how to prepare for an IVF cycle and IVF success rate by age. Meet Dr. Lata Goyal and Dr. Ankita Bansal Goyal, see the full consultant team, browse all clinical departments, check our insurance and scheme empanelments, or book a consultation here.
"One egg collection, several embryos, several chances. Thinking in those terms rather than cycle by cycle changes both the odds and the experience."
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