How this IVF due date calculator works
Obstetric dating counts pregnancy from the last period, about 280 days before the due date. An embryo at transfer is already a known number of days past fertilisation, so that age is subtracted from 280.
Day 5 blastocyst: due date = transfer date + 261 days
Day 3 cleavage-stage embryo: due date = transfer date + 263 days
Gestational age on transfer day = 2 weeks 5 days (day 5) or 2 weeks 3 days (day 3)
Current gestational age = today − transfer date + 19 days (day 5) or + 17 days (day 3)
- Day 5: 280 − 19 = 261. A blastocyst is five days past fertilisation, and fertilisation is counted as about 14 days after the last period, so the transfer day is 2 weeks 5 days (19 days) into the pregnancy.
- Day 3: 280 − 17 = 263. A cleavage-stage embryo is three days past fertilisation, which makes the transfer day 2 weeks 3 days (17 days).
- The embryo's age is not your pregnancy's age. The transfer day already counts as roughly two weeks pregnant, which is why the due date is not simply 40 weeks after transfer.
- All date maths uses whole UTC calendar days, so a clock change can never shift a result by a day.
- Only day 3 and day 5 are supported. Other stages use different arithmetic, so the calculator does not guess at them.
Frozen transfers, fresh transfers and ERA
If you had a frozen embryo transfer and no adjustment was made to the timing, the same embryo-age arithmetic applies: the embryo's stage matters, not whether it was frozen. This is the convention most clinics use when no endometrial receptivity array (ERA) or other timing adjustment has changed the transfer day.
If your clinic used an ERA or another protocol that moved the transfer relative to your cycle, the due date should be based on the timing your clinic used. In that situation, this calculator can be a day or more out, and the clinic's date should win.
A fresh transfer of a day 3 or day 5 embryo uses the same embryo-age rule. The date your clinic gives you takes priority because it reflects the exact stage recorded in the laboratory.
When the result may not apply
This calculator is a single formula with two inputs. Several situations fall outside it, and in each of them your reproductive centre's date is the one to use:
- A day 6 blastocyst or another embryo stage. One extra day of development changes the subtraction, so the arithmetic here does not apply.
- More than one embryo transferred. A multiple pregnancy may be dated and monitored differently, and the clinic will give you the date it is using.
- A clinic-specific dating convention. Some centres adjust the date after an early scan. A scan measurement can revise the date you were given.
- An uncertain transfer date. If you are unsure which date was the transfer, check your clinic letter before relying on any calculation.
If your clinic's due date differs from this one, do not change it yourself. Ask your clinic which date they are using and why.
Frequently asked questions
Why does a day 5 transfer add 261 days?
A pregnancy is dated from the last period, which is about 280 days before the due date. A day 5 blastocyst is already 5 days past fertilisation on transfer day, and obstetric dating counts fertilisation as roughly 14 days after the last period. That makes the transfer day about 19 days into the pregnancy, so the due date is 280 minus 19, or 261 days, after transfer.
Does a frozen embryo transfer change the due date?
If you had a frozen embryo transfer and no adjustment was made to the timing of the transfer, the same embryo-age arithmetic applies. If your clinic used an endometrial receptivity test or another protocol that shifted the transfer, the due date should be based on the timing your clinic used, not on this calculator.
What if I had a day 6 blastocyst?
This calculator supports day 3 and day 5 transfers only. A day 6 blastocyst is one day further on in development, so the arithmetic is different. Use the due date your reproductive centre gave you rather than adjusting this result yourself.
Can I use this after a fresh embryo transfer?
The embryo-age part of the calculation is the same for a fresh transfer of a day 3 or day 5 embryo. The date your clinic gives you takes priority, because it also reflects the exact embryo stage and the clinic's own dating convention.
Sources and disclaimer
Sources used for the method and the notes on this page:
This page is informational only and is not a medical device. It does not create a clinician–patient relationship and must not be used to make medical decisions. Your reproductive centre's dating takes priority over any result on this page.
Your dates are processed only in this browser tab. Nothing is uploaded, stored or shared.
Related tools
The conception date calculator works in the other direction, from a due date or last period. If you have two hCG results, the hCG doubling calculator shows the interval and doubling time.