Short answer: A frozen embryo is thawed and transferred back to the uterus in a later cycle, either in the body's own cycle or with hormonal medication that controls the timing. Pregnancy tests are done 13–16 days after the transfer. Neither method has been shown to result in more live births, but studies in the field are small.
How does a frozen embryo transfer work?
A frozen embryo is not transferred in the same cycle as the egg retrieval. The embryo is thawed before the transfer and placed back in the uterus in a later treatment cycle. You can read how the entire IVF process looks before freezing in IVF step by step.
According to the Karolinska University Hospital's reproductive medicine clinic, the pregnancy test is done 13–16 days after the transfer. If there are frozen embryos, they must be used before a new fresh treatment starts, and they can be used until the date when the frozen storage period expires. How to read the test is described in the article about pregnancy tests.
Natural cycle or hormone treatment – does the choice matter?
The uterine lining needs to be ready when the embryo is transferred back. This can be arranged in a few different ways, and Cochrane has compiled 32 randomized studies with 6,352 women where the setups were compared. Most of the women had regular ovulation, so the results say little about those who have irregular or absent ovulation.
The setups compared are natural cycle (the body's own ovulation controls the timing), modified natural cycle (where ovulation is triggered with an hCG injection), hormone treatment (HT), and hormone treatment with the addition of a GnRH agonist, which suppresses the body's own cycle.
| Comparison | Live births, odds ratio (95% CI) | Number of women (studies) |
|---|---|---|
| Natural cycle versus HT | 1.18 (0.67–2.08) | 233 (1) |
| Natural cycle versus HT + GnRH agonist | 0.89 (0.58–1.36) | 400 (2) |
| Natural versus modified natural cycle | 0.97 (0.65–1.45) | 442 (3) |
| Modified natural cycle versus HT | 1.26 (0.90–1.77) | 1,189 (2) |
| HT versus HT + GnRH agonist | 0.92 (0.71–1.19) | 1,132 (5) |
An odds ratio close to 1 means that the groups had approximately the same chance of a live birth. All confidence intervals in the table include 1, so no comparison pointed to a better setup. Cochrane assesses the reliability as low to moderate, because the studies are small and many important outcomes were not reported. It is therefore the clinic that chooses the setup based on your cycle and their routines, not a method that has been proven to be best.
Freeze all embryos immediately – what does it result in?
Sometimes all embryos are frozen after the egg retrieval (freeze-all) instead of transferring a fresh embryo. The Cochrane review on this includes 15 randomized studies, eight of which are included in the main analysis with 4,712 women.
The chance of live births did not differ clearly: odds ratio 1.08 (95% CI 0.95–1.22). If 58% get children with conventional treatment, the corresponding figure with freeze-all is between 57% and 63%. The risk of ovarian overstimulation (OHSS) was lower with freeze-all, approximately 1% compared to 3%. At the same time, more cases of high blood pressure during pregnancy (Peto OR 2.15; 95% CI 1.42–3.25) and more children who were large for their gestational age (Peto OR 1.96; 95% CI 1.51–2.55) were seen. These safety outcomes are based on three studies with 3,940 women and are assessed as low reliability.
The transfer is also postponed with freeze-all, so the time until a possible pregnancy will likely be longer. How long the luteal phase is and how progesterone works in a natural cycle is described in the article about the luteal phase and progesterone.
Does a transfer count as a new IVF attempt?
According to Karolinska, a transfer with frozen embryos does not count towards the three IVF treatments with egg retrieval, as long as pregnancy and children have not been achieved. The woman must be under 40 years old at each treatment occasion, where treatment occasion means the start of medication. The age limit for embryo transfer is the same as for eggs. The rules apply to the described healthcare region, so ask your clinic what applies to you.
Frequently asked questions
When can I take a pregnancy test after a transfer? At Karolinska, the test is done 13–16 days after the transfer. The clinic gives you a date for the test.
Do fresh embryos give better chances than frozen? Cochrane found no clear difference in the cumulative chance of live births between freeze-all and conventional treatment. The differences were instead in OHSS, blood pressure, and the children's weight.
Do I have to use frozen embryos before new treatment? Yes, according to Karolinska, frozen embryos must be used before a new fresh treatment starts, and they can be used until the end date of the frozen storage.
Read also
- IVF step by step – from hormone stimulation to transfer
- How early can you rely on a pregnancy test?
- The luteal phase and progesterone – length, testing, and treatment
General information, not medical advice. Consult healthcare about your own situation.