Why IVF due dates are different
Natural-cycle dating has a built-in guess: nobody knows exactly which day ovulation happened. With IVF, that guess disappears. The fertilization date is known, the embryo's age at transfer is known, and so the pregnancy can be dated precisely. An IVF due date is the gold standard that other dating methods are measured against.
The convention works like this. In a natural cycle, fertilization happens about 14 days after the last period started, so a pregnancy is "2 weeks along" at fertilization. An embryo transferred on day 5 after fertilization is therefore treated as 19 days of pregnancy already (14 plus 5). A day 3 embryo is treated as 17 days (14 plus 3).
The two formulas
Day 5 (blastocyst) transfer: due date = transfer date plus 261 days. That is 280 minus the 19 days already counted.
Day 3 (cleavage-stage) transfer: due date = transfer date plus 263 days. That is 280 minus the 17 days already counted.
The 2-day difference between the formulas is exactly the 2-day difference in embryo age. If you are unsure which transfer you had, blastocyst (day 5 or 6) transfers are by far the most common today; day 3 transfers are less common but still used.
Worked example: day 5 transfer
Suppose your day 5 transfer was February 1, 2026. Add 261 days: February 1 plus 261 days is October 20, 2026. That is the estimated due date.
Sanity check it as gestational age. On transfer day you were already "19 days pregnant" by the dating convention, which is 2 weeks 5 days. Six weeks later, on March 15, you would be 8 weeks 5 days. And on October 20 you would be exactly 40 weeks 0 days. The arithmetic closes cleanly, which is the signature of IVF dating.
Worked example: day 3 transfer
Suppose your day 3 transfer was February 1, 2026. Add 263 days: February 1 plus 263 days is October 22, 2026. Two days later than the day 5 example, exactly as expected.
The gestational age on transfer day is 17 days, or 2 weeks 3 days. Notice that the due date for the day 3 transfer lands two days after the day 5 one for the same transfer date. That gap never changes: it is baked into the embryo ages.
What about day 6 transfers and frozen embryos
A day 6 blastocyst is dated the same as a day 5: it is still a blastocyst, so use the day 5 formula (transfer plus 261 days). The one-day difference in development is smaller than the dating convention's rounding and providers standardize on day 5 math.
Frozen embryo transfers use the same formulas. Whether the embryo was transferred fresh or after freezing, what matters is the embryo's age in days at transfer, not the calendar gap between retrieval and transfer. A frozen day 5 blastocyst transferred February 1 gets the same October 20 due date as a fresh one.
For donor-egg or gestational-carrier cycles, the math is identical: date from the transfer, using the embryo's age. The pregnancy calculator has an IVF mode that runs this for day 3 and day 5 transfers automatically.
How accurate are IVF due dates
More accurate than any other method, because the single biggest source of error, the unknown ovulation day, is removed. Studies comparing IVF dates to first-trimester ultrasound dates find they agree within a few days in the vast majority of cases.
That said, the due date is still an estimate of a 40-week point, not an appointment. Full term runs 39 to 40 weeks, and only about 1 in 20 babies arrives on the exact date. The value of the IVF date is that your provider can measure growth against a reliable baseline, which makes every later scan more informative.