How IVF due dates are calculated
With IVF the moment of fertilisation is known, so the calculator does not need to estimate ovulation. It treats egg retrieval (fertilisation) as conception and adds 266 days. When you enter a transfer date, it subtracts the embryo’s age, because a day-5 embryo was fertilised five days before transfer:
| Date entered | Days added | Pregnancy on that day |
|---|---|---|
| Egg retrieval | 266 | 2 weeks 0 days |
| Day-3 embryo transfer | 263 | 2 weeks 3 days |
| Day-5 blastocyst transfer | 261 | 2 weeks 5 days |
| Day-6 blastocyst transfer | 260 | 2 weeks 6 days |
Example: a day-5 blastocyst transfer on April 14, 2026 gives an estimated due date of December 31, 2026. The equivalent “last period” date — from which weeks are counted — is March 26, 2026.
Fresh and frozen transfers
For a frozen embryo transfer (FET), use the date of the transfer and the embryo’s age when it was frozen. A day-5 blastocyst frozen and transferred months later is still dated as day 5 on transfer day. Your clinic may give you its own due date; if it differs by a day, it is usually because of whether the transfer day is counted as day 0 or day 1.
Why IVF due dates rarely change
Because the embryo’s age is known precisely, IVF due dates are generally not adjusted after ultrasound scans, unlike LMP-based dates. The usual uncertainty about when labour starts still applies, though: most babies, including IVF babies, arrive within a couple of weeks of the due date. Once you know your dates, the Pregnancy Week Calculator and Pregnancy Timeline work with your due date to follow the pregnancy week by week.