The formula
How to calculate iVF due date
IVF dating is more precise than natural conception dating, because the exact age of the embryo is known. The due date is calculated from the transfer rather than from a last menstrual period, which may not be relevant at all.
The 266 days is the average length of a pregnancy from conception, as opposed to the 280 days counted from the last period. The two-week difference is why 14 days is added when converting to gestational age.
What to enter:
- Embryo age at transfer (days) — 3 for a cleavage-stage embryo, 5 for a blastocyst
- Days since the transfer (days)
Everything recalculates as you type, and the numbers in the address bar update with it, so a link to this page carries your figures with it.
Consistency in how the underlying measurements are taken matters more than precision to the last decimal — measuring at the same time of day, in the same state of rest or hydration, makes readings taken weeks apart genuinely comparable in a way that a single very precise reading on its own is not.
Why iVF due date matters
The formula behind iVF due date comes from published research rather than being invented for this page, and it is presented here exactly as it appears in the literature it is drawn from — the value of a calculator is applying it correctly and instantly, not reinventing it.
Because the calculation depends on a formula rather than a device, it is useful anywhere a scale, a tape measure or a stopwatch is available, without needing specialist equipment — which is also exactly why it should be treated as an estimate rather than a clinical measurement.
A single number like this is most useful placed in context rather than read in isolation. Reference ranges exist because bodies vary considerably and still count as healthy across a wide spread of values, so a result sitting outside a commonly quoted range is a prompt to look more closely rather than a verdict on its own. The same figure, tracked consistently over time, tends to say more than any single reading ever will.
A calculator like this one is frequently used to prepare a specific number for an upcoming appointment, so that a description like "I think it has changed" can be replaced with an actual figure and a date it was measured on. Clinicians and trainers generally find a concrete number, however approximate, more useful to work from than a general impression.
Worked example
Here is the calculation with the starting values:
- Embryo age at transfer: 5 days
- Days since the transfer: 40 days
That gives:
- Days until the due date: 221 days
- Gestational age, completed weeks: 8 weeks
- Plus days: 3 days
Those starting figures exist only to show the calculation working, not as a target or a reference value to compare yourself against — replace them with your own measurements in the calculator above and the same formula applies exactly as shown.
Reading the result
A day-5 blastocyst transferred today puts you at 2 weeks and 5 days gestational age immediately, which surprises many people. The clock started before the transfer, not on it.
Where this goes wrong. Frozen embryo transfers use the same arithmetic based on the embryo's age at freezing, not the time spent in storage. Your clinic's dating is the authority — this page is general information.
As with any general-purpose formula, the result here is an estimate derived from population averages, not a measurement specific to you — a reasonable starting point for a conversation with a qualified professional, not a replacement for one. Nothing on this page is medical advice.
Because LMP dating assumes ovulation on day 14 of a 28-day cycle. IVF knows the actual date of fertilisation, so it removes that assumption and the error that comes with it.
Yes, generally the most accurate dating available, since conception timing is known exactly. A first-trimester scan usually confirms it within a couple of days.
The answer it gives you is days until the due date. With 5 days embryo age at transfer and 40 days days since the transfer, that comes to 221 days. Change any field and the figure moves with it.
It uses a published formula applied exactly as defined, so the arithmetic itself is precise — but the formula is a population-level estimate, and individual results can vary from it for reasons the formula has no way to account for. Treat the output as a reasonable estimate rather than a measured or diagnostic value.
For most people, yes — a single reading is a snapshot, while the same measurement repeated consistently over weeks or months shows a trend, which is generally more informative than any one figure taken in isolation.
Small differences between calculators usually come down to which published formula or reference dataset each one uses — several equations in this area exist in more than one accepted version. This page states the specific formula used in the section above, so it is possible to check directly whether another source is using the same one.