You won't know your natural FET transfer day at the start, because it depends on when you ovulate. The transfer isn't on ovulation day. Your clinic counts a set number of days from ovulation, your LH surge or a trigger shot, based on your embryo's age. Your part is to go to each scan and blood test, and to do any home tests the way your clinic asks. Your clinic's plan comes first, and the warning signs are near the end.
What is a natural FET cycle?
In a natural FET cycle, the follicle that grows in your ovary each month makes estrogen. A follicle is the small sac that holds an egg. That estrogen thickens the lining of your uterus.
After you ovulate (release the egg), your body makes progesterone. It comes from the corpus luteum, the part of the ovary left behind after ovulation. Progesterone gets the lining ready for an embryo. Your frozen embryo is then thawed and placed in your uterus on a day that matches its age. For the full picture, see frozen embryo transfer.
There are two main kinds:
- True natural cycle. No medicine is used to make you ovulate. The clinic tracks ovulation with ultrasound scans, and may use blood tests for hormones such as LH, home urine LH tests, or a mix.
- Modified natural cycle. A trigger shot of hCG helps time ovulation, so the transfer is easier to plan.
Who is a natural FET for?
A natural protocol needs a regular menstrual cycle. Many clinics only offer it to people with regular periods or regular ovulation.
If you don't ovulate regularly, there are other options. Many clinics let patients use medicine to help them ovulate, most often letrozole. A medicated or programmed FET is another path.
What does the research show?
Natural cycles use fewer medicines and can cost less. But dates are harder to plan, they need more monitoring, and more cycles are cancelled.
In January 2026, The BMJ published a randomized trial from 24 fertility centers in China. It included 4,376 women aged 20 to 40 who had regular cycles and were having one frozen blastocyst transferred. Half were assigned a natural-ovulation cycle and half a medicated cycle.
- Healthy live birth (one baby born at 37 weeks or later, at a normal weight, without major birth defects): 41.6% with natural cycles and 40.6% with medicated cycles, which the trial counted as similar.
- Pre-eclampsia, a serious blood pressure problem in pregnancy: among those whose pregnancy was seen on ultrasound, 2.9% after natural cycles and 4.6% after medicated cycles.
- Cancelled cycles (on the first try at preparing the lining): 16.2% of natural cycles and 11.5% of medicated cycles.
Earlier studies that looked back at past pregnancies also found fewer blood pressure problems after natural cycles. The trial's authors say their results may not apply to people with ovulation problems, a thin lining, repeated failed transfers or scarring inside the uterus. Its clinics also followed their own practice on progesterone in natural cycles.
Natural FET timeline: the checkpoints
| Step | What happens | Your checkpoint |
|---|---|---|
| Period starts | This is day 1 of your cycle | Ask your clinic when to call in |
| Scans | An ultrasound checks your growing follicle | Go to every scan the clinic books |
| Ovulation tracking | Blood tests, home urine LH tests or both, as your clinic decides | Follow the testing plan and report results as asked |
| Ovulation check | An ultrasound may confirm the follicle released its egg | Wait for the clinic to confirm |
| Transfer date set | The clinic counts days from ovulation, your surge or the trigger | Write down the date and time |
| Transfer day | The embryo is thawed that morning and placed in your uterus | Follow the clinic's arrival steps |
| Pregnancy test | A blood test, about 9 to 14 days after transfer | Ask your clinic for the exact day |
Monitoring and ovulation tests
Monitoring starts with a vaginal ultrasound that measures the follicle as it grows. You may need more than one scan as it gets bigger. Your clinic may also check hormone levels with blood tests.
Some clinics ask you to use home ovulation tests too. These strips measure LH in your urine. LH is a hormone that rises quickly just before you ovulate, and that rise is called the LH surge. If your clinic uses home tests, it will tell you when to test and what to report. If a result is unclear, call, because the clinic may want another scan.
Why the timing matters
An embryo and your lining need to be in step. The clinic counts a set number of days from ovulation, your surge or your trigger. An embryo frozen at the blastocyst stage is transferred later than one frozen earlier. In the BMJ trial, a blastocyst was transferred five days after ovulation, six days after the LH surge began, or seven days after an hCG trigger.
So if you think you missed a surge or a test, tell the clinic rather than guess.
When the cycle doesn't go as planned
Sometimes the follicle doesn't grow enough. Sometimes the surge or ovulation doesn't come when the plan needs it. In one study, these were the main reasons natural cycles were cancelled.
An ultrasound may also show the follicle didn't release its egg. Then the clinic may cancel the transfer or give you hormones to support the cycle.
A cancelled cycle can be disappointing after weeks of tests. Your embryo is not thawed until transfer day, so ask your clinic when you can try again and whether a different plan would fit better. Read more in cycle cancellation.
Modified natural FET: adding a trigger shot
When your follicle is big enough, you may get a trigger shot of hCG, a hormone that helps time when you ovulate. The clinic then counts days from the shot. This helps coordinate ovulation with the clinic's schedule and needs fewer checks than a true natural cycle. In a U.S. survey, most clinics that offer natural FET timed it with an hCG trigger.
If your own LH surge has already started, your clinic may time the transfer from it instead. In the BMJ trial, the trigger was given only when the follicle was big enough and no surge had begun.
Take the trigger at the exact time your clinic gives you, because the transfer day is counted from it. Set an alarm, and if you miss it or are unsure, call right away.
Do you need progesterone in a natural FET?
This differs between clinics, and the answer can differ for true natural and modified natural cycles.
A randomized trial at two university clinics in Sweden, published in Human Reproduction in 2022, included 488 women with regular cycles. Their transfers were in true natural cycles, timed with a home urine LH test and no trigger. Half used vaginal progesterone tablets from transfer day, and half used nothing. Live births were 34.2% with progesterone and 24.1% without. The trial was not blinded, and it did not include trigger cycles, so its results can't be assumed to apply to modified natural cycles.
If your clinic prescribes progesterone, ask which form you will use and for how long. Then keep taking it until the stop date your clinic gives you.
After the transfer
Keep taking any medicines you were given. Your clinic will check for pregnancy with a blood test. If it is positive, you will have more tests and a first ultrasound. Read more in after embryo transfer.
Tips for a natural FET cycle
- Ask your clinic how it will track ovulation, and which home tests to buy, if any.
- Keep a simple log of test results and scan dates.
- Ask who to call if a test turns positive on a weekend.
- If you have a trigger shot, check the time twice and set two alarms.
- Before you leave each visit, confirm your next test and appointment.
When to get help
Call your clinic right away, day or night, if you have:
- A fever of 100.4°F (38°C) or higher.
- Bleeding from your vagina, or a brown, watery discharge. Light spotting can happen after a transfer. Still, let your clinic decide what it means.
- Belly or pelvic pain that is severe, gets worse, or isn't eased by the pain relief your clinic recommended.
- Pain and bloating in your belly, or bloating that is getting worse.
- Nausea and vomiting, nausea that won't go away, or you can't keep fluids down.
- Pain, redness or warmth in an arm or leg.
- Blood in your urine, or pain when you pee or poop.
Tell your clinic at your next contact about other milder symptoms that are new, don't go away or slowly get worse, such as sore breasts or mild cramps.