Had a frozen embryo transfer? Enter your transfer date and the embryo's age at freezing to get an estimated due date using the same formula fertility clinics trust.
What Is the FET Due Date?
An FET due date is the estimated delivery date for a pregnancy that began with a frozen embryo transfer, calculated from the transfer date and the embryo's age when it was frozen. Freezing pauses development but never resets it: a Day 5 blastocyst is still 5 days old at transfer, so you add 261 days to the transfer date.
The idea that cold storage could 'reset' an embryo is one of the most common worries patients bring to their first FET. Modern vitrification — an ultra-fast freezing technique — essentially suspends the embryo in time. Whether it waited in the freezer for three weeks or three years, it wakes up at exactly the developmental stage it went to sleep in, and the due date math treats it accordingly.
Frozen transfers now account for a large share of IVF births, and their success rates match or exceed fresh transfers in many clinics. Part of the reason is timing: with no egg retrieval in the same month, your body is not recovering from stimulation hormones, and the clinic can place the embryo into a calm, well-prepared lining. Your due date, though, needs none of that context — just the transfer date and the embryo's frozen age.
The rise of FET is one of the biggest shifts in modern fertility care. "Freeze-all" cycles — where every viable embryo is banked and the transfer happens a month or more later — are now routine, chosen to let ovaries recover from stimulation, to allow time for preimplantation genetic testing, or simply to wait for the best uterine conditions. Embryo banking for fertility preservation follows the same path: eggs or embryos frozen in your twenties can become a pregnancy in your thirties, dated from the future transfer day with the embryo's frozen age — never from the freezing day.
How to Use the FET Due Date Calculator
- Find your transfer date. This is the day the thawed embryo was placed, listed in your clinic portal or FET cycle paperwork — not the freezing date.
- Choose the embryo's age at freezing. Day 5 blastocyst is the most common choice; Day 3 and Day 6 are also routine. Your embryology report states the day.
- Enter both into the calculator above. Select the transfer date and the matching embryo age option.
- Read your timeline. The result shows your estimated due date, gestational age, and week-by-week milestones from implantation through delivery.
- Confirm against your clinic's chart. The formula is universal, so your clinic's official date should match within a day. If yours differs, ask which embryo age they used — that is almost always the explanation.
How the Calculation Works
The logic mirrors fresh IVF dating exactly, because biologically the embryo does not know it was ever frozen. The 266-day count is the average pregnancy length measured from fertilization, and the embryo's age — 3, 5, or 6 days — is subtracted so those days of development are counted once. Time in the freezer simply does not exist on the embryo's clock.
The one thing that does change between FET protocols is how transfer day itself is chosen. In a medicated FET cycle, estrogen builds the lining on a fixed schedule and progesterone is started a set number of days before transfer, giving the clinic exact control. In a natural FET cycle, the clinic tracks your own ovulation and times the transfer to it. Both paths land on the same formula — the cycle type only determines the transfer date, not how you count from it.
Worked Example
Say your clinic transferred a frozen Day 5 blastocyst on Monday, March 2, 2026. Applying the formula: March 2, 2026 + 261 days = Wednesday, November 18, 2026. That Wednesday is your estimated due date, marking 40 weeks of gestational age — even though the embryo itself was created months or years earlier.
For a frozen Day 6 blastocyst transferred on Monday, June 1, 2026, the calculation is June 1, 2026 + 260 days = Tuesday, February 16, 2027. Notice the due date lands one day earlier than a Day 5 would, because the older embryo had one extra day of development before freezing. Small shift, same rule.
The cycle type shows up in the calendar around these dates, not in the math. In a medicated FET, you might start estrogen in early February, have your lining checked mid-month, begin progesterone about five days before the March 2 transfer, and continue both hormones until roughly week 10 of pregnancy. In a natural FET, the only "medication" is timing: the clinic tracks your LH surge and schedules the transfer a fixed number of days later. Either way, the due date formula only ever sees the transfer date and the embryo's age.
How Accurate Is It?
FET dating is just as reliable as fresh IVF dating — the two inputs are known facts, not estimates. Research suggests transfer-derived due dates line up with first-trimester ultrasound dating within a few days for the large majority of pregnancies. The exact transfer day is documented, the embryo age is documented, and implantation timing varies only by a day or two either way.
The main sources of fuzziness are the same as any pregnancy: only about 1 in 20 babies arrives on the due date itself, and implantation can take 1–5 days after transfer. Medicated FET cycles add a small wrinkle — progesterone supplementation can mimic early pregnancy symptoms, tempting patients to read meaning into them before the beta test. Let the blood test and the 6- to 7-week ultrasound settle the timeline rather than symptoms.
Two things patients often wonder about do not affect dating at all: the embryo's grade and the number of years it spent frozen. A top-graded blastocyst and a fair-graded one share the same developmental age, and an embryo frozen in 2021 is no "older" at transfer than one frozen last month. Grading predicts the chance of implantation, and storage time predicts nothing measurable — neither one moves the due date by a single day.
When to Talk to Your Doctor
Contact your clinic promptly for heavy bleeding (soaking a pad hourly), severe pelvic pain — especially one-sided pain with dizziness, which can signal an ectopic pregnancy — fainting, or fever over 100.4°F. FET cycles carry a much lower risk of ovarian hyperstimulation than fresh cycles, but if your FET followed a recent retrieval, report rapid bloating or weight gain and breathing difficulty the same day.
If you are on a medicated cycle, do not stop your estrogen or progesterone on your own, even if you bleed — medication is usually continued until the clinic confirms the outcome and gives explicit instructions. Later in pregnancy, the standard red flags apply: regular contractions before 37 weeks, fluid leakage, reduced fetal movement, severe headache with vision changes, or any bleeding. Your team expects these calls; make them early.
One reassuring note for the anxious weeks: light spotting after a frozen transfer is common and often related to the progesterone support or the catheter passing through the cervix — it does not by itself signal a failed cycle. Only your beta hCG results can answer that question, so try to let the blood test do its job before drawing conclusions from symptoms.
Once your dates are set, you can track each week with our pregnancy week-by-week guide, and compare protocols in our article on FET vs. fresh transfer.
This calculator provides estimates for educational purposes and is not medical advice. Always confirm with your healthcare provider.