Quick answer
Add 261 days to a day-5 transfer or 263 to a day-3 transfer. Because conception timing is documented, IVF dating is more precise than LMP-based dating.
The one thing IVF dating gets right
Conventional due-date calculation carries an assumption it cannot verify: that ovulation occurred on day 14 of a 28-day cycle. For many people it did not, which is why LMP-based dates are often revised after a dating scan.
Where natural conception is the plan, the same calendar arithmetic runs in reverse: an ovulation calculator estimates the fertile window from the last menstrual period and the actual cycle length, rather than assuming ovulation on day 14.
IVF removes that uncertainty. Fertilisation happens in the laboratory on a recorded date, and the embryo's age at transfer is documented. There is no need to estimate when conception occurred, because it is known. This makes IVF due dates one of the few situations where the initial estimate rarely needs revising.
Use this guide with the right tool: Open the IVF Due Date Calculator for an estimated due date calculated from an embryo transfer date and embryo day. If the question shifts, compare it with the Reverse Due Date Calculator or the Pregnancy Due Date Calculator.
Embryo day and the arithmetic
The calculation works backwards from the 280-day gestational model. Gestational age runs about two weeks ahead of conception, so from the fertilisation date the remaining span is 280 − 14 = 266 days. Subtract the embryo's age at transfer to get the interval from transfer to due date.
A day-5 transfer therefore adds 261 days; a day-3 transfer adds 263; a day-6 transfer adds 260. Getting the embryo day right matters — a two-day error is small in absolute terms but shifts every subsequent gestational-age milestone by the same amount.
Frozen transfers and donor cycles
For a frozen embryo transfer the same arithmetic applies, because what matters is the embryo's developmental age at transfer, not how long it was stored. Storage time does not enter the calculation at all.
Donor egg cycles follow the recipient's transfer date and the embryo day, not the donor's cycle. The gestational clock starts with the embryo, so the recipient's own menstrual history is not part of the calculation.
Precision is not certainty
A more precise starting point produces a more reliable estimate, but it does not make delivery predictable. The distribution of actual gestation lengths is much the same regardless of how conception was dated, so births still occur across a window of weeks around the estimated date.
Use the date for planning — leave, appointments, preparation — and treat it as a well-founded midpoint. Your clinic's own date is the one to record, and it should agree closely with this calculation if the transfer date and embryo day are entered correctly.
Worked example
A day-5 embryo transferred on 16 August 2026 gives a due date of 4 May 2027, 261 days later, with an implied LMP of 28 July 2026. Enter your transfer date and embryo day to date the pregnancy from transfer.
One transfer date, three embryo days
The worked example dates a day-5 transfer made on 16 August 2026. Running the same transfer date at the other two common embryo ages shows how much of the final answer is carried by that one input — and how the three-day spread at the start is still a three-day spread at every milestone afterwards.
Had the embryo transferred on 16 August 2026 been a day-3 embryo instead, the interval to add would be 263 days and the estimated due date 6 May 2027. The implied last menstrual period for gestational-age purposes would be 30 July 2026, and the pregnancy would have been dated at 2 weeks 3 days on the transfer day itself. A day-6 embryo on the same date adds 260 days, giving 3 May 2027, with an implied LMP of 27 July 2026 and a gestational age at transfer of 2 weeks 6 days. The day-5 case sits between them: due 4 May 2027, implied LMP 28 July 2026, dated 2 weeks 5 days at transfer.
Three due dates, three days apart end to end. That sounds negligible, and for the birth itself it is — the window of likely delivery dates is weeks wide, as the earlier section notes. But every dated milestone in between inherits the same offset, because all of them are counted from the implied LMP. The twelve-week point falls on 20 October 2026 for the day-5 pregnancy, on 22 October for the day-3, and on 19 October for the day-6. A screening appointment booked from the wrong embryo day is not wrong by a rounding error; it is wrong by exactly the embryo-day error, reproduced faithfully at every step.
The three timelines stay parallel all the way. On 1 January 2027, the day-3 pregnancy is dated at 22 weeks 1 day, the day-5 at 22 weeks 3 days, and the day-6 at 22 weeks 4 days. Nothing converges and nothing diverges; the initial difference is preserved intact because it was never an approximation to begin with. That is the practical meaning of the precision this guide attributes to IVF dating. Where the inputs are documented, the arithmetic transmits them exactly — including when one of them is wrong.
Which returns to the mistake listed below, now with its size attached: entering the wrong embryo day costs one to three days, not a reassessment. The check that catches it costs nothing either — compare the implied LMP or the gestational age at transfer against the clinic's record, where the same figures were derived from the same documented dates. If they disagree, one of the two records has the embryo day or the transfer date wrong, and the arithmetic on both sides is good enough to prove it.
Common mistakes to avoid
- Using LMP-based arithmetic for an IVF pregnancy. Transfer-based dating is more reliable because conception timing is documented rather than assumed.
- Entering the wrong embryo day. Day 3, 5 and 6 transfers give different intervals. Take it from the clinic record.
- Counting storage time for a frozen transfer. Only the embryo's developmental age at transfer matters. Storage duration is not part of the calculation.
Use the right calculator
IVF Due Date Calculator dates pregnancy from transfer date and embryo day, Reverse Due Date Calculator works back from gestational age, and Pregnancy Due Date Calculator starts from LMP.
Frequently asked questions
Why is the pregnancy already two weeks along at transfer?
Gestational age is counted from the conventional LMP point, which precedes conception by about two weeks. The clock starts before the transfer by definition.
Does a frozen transfer change the date?
No. The arithmetic uses the embryo's developmental age at transfer, regardless of how long it was frozen.
Should this replace the clinic's date?
No. Record the clinic's date. This calculation should match it closely and helps you understand how it was derived.