How is an IVF due date calculated?

The formula

A pregnancy runs roughly 266 days from conception. In IVF, day 0 of embryo development is the day of fertilisation, so the calculation subtracts however many days the embryo had already developed before transfer.

A day 5 blastocyst has used five of those days, leaving 261. A day 3 embryo has used three, leaving 263. Egg retrieval and IUI are both day 0, so they carry the full 266.

• Day 3 transfer: transfer date + 263 days
• Day 5 transfer: transfer date + 261 days
• Day 6 transfer: transfer date + 260 days
• Day 7 transfer: transfer date + 259 days
• Egg retrieval or IUI: date + 266 days

Fresh and frozen transfers

The rule does not change for a frozen embryo transfer. What matters is the age the embryo had reached when it was frozen, because development pauses in storage and resumes on thawing.

A blastocyst frozen on day 5 and transferred a year later is still a day 5 embryo for dating. Time spent frozen does not enter the calculation at all.

Why you are already two weeks pregnant on transfer day

Gestational age is counted from a notional last menstrual period, roughly two weeks before conception. That convention applies to IVF pregnancies too, so the clock starts about two weeks ahead.

On the day of a day 5 transfer, the gestational age works out to 2 weeks and 5 days. It looks strange the first time you see it, but it is what keeps IVF pregnancies on the same 40-week scale as everyone else, so screening windows and appointment timings line up normally.

Getting the inputs right

The only figure worth double-checking is the embryo's age at transfer, which your clinic records. The difference between a day 5 and a day 6 blastocyst is one day on the due date; between day 3 and day 5 it is two.

Where your clinic has given you a due date, use theirs. Conventions differ slightly between clinics, and yours is the one your care will be scheduled against.