IVF Due Date Calculator

Dating a pregnancy after IVF (in vitro fertilisation) is more precise than dating a natural one, for a simple reason: the fertilisation date is known to the day. Natural conception dating leans on Naegele’s Rule, which assumes ovulation happened on day 14 of a 28-day cycle. Plenty of people do not oblige.

That is also why the American College of Obstetricians and Gynecologists (ACOG) says an IVF due date should come from the embryo’s age and the transfer date, rather than from a period date or a scan (Committee Opinion 700, 2017).

The formulas

A full pregnancy runs 266 days from fertilisation, and the embryo has already used some of them by transfer day:
Day-3 embryo: due date = transfer date + 263 days.
Day-5 blastocyst, the most common transfer now: due date = transfer date + 261 days.
Day-6 blastocyst, common in frozen cycles: due date = transfer date + 260 days.

Why they differ. A day-5 blastocyst is already five days old when it goes in, so 266 − 5 = 261. A day-3 embryo is three days old, 266 − 3 = 263, and a day-6 one gives 260. That is the entire difference.

Where the 2-week head start comes from

Pregnancy is counted in gestational weeks, and by convention gestational week 0 is the first day of the LMP (last menstrual period), roughly two weeks before fertilisation. So the moment an egg is fertilised, the pregnancy is already “2 weeks” old on paper.

That gives an equivalent LMP date, which is what a clinic will use on every scan and chart:
Day-5 transfer: equivalent LMP = transfer date − 19 days (5 days of embryo age + 14 days of convention).
Day-6 transfer: transfer date − 20 days. Day-3 transfer: transfer date − 17 days.

Check it against the standard 40-week pregnancy: transfer − 19 + 280 = transfer + 261, which is the due date above. The two ways of counting agree.

Gestational age on transfer day is 2 weeks and 5 days for a day-5 blastocyst (2 weeks 6 days for day 6, 2 weeks 3 days for day 3). It is not “5 days” and it is not zero.

Worked example

Day-5 blastocyst transfer on 15 March 2025.
Due date = 15 March + 261 days = 1 December 2025.
Equivalent LMP = 15 March − 19 days = 24 February 2025.
Cross-check: 24 February + 280 days is also 1 December.
On transfer day the pregnancy reads 2 weeks 5 days.
The same date with a day-6 blastocyst would be due 30 November, and with a day-3 embryo 3 December.

Milestones, counted from the transfer date

The numbers are days after the transfer. Trimester lines are a convention rather than a biological event, and you will see the first trimester quoted as ending anywhere from 12 to 14 weeks. This page uses ACOG’s lines, the first trimester running to 13 weeks 6 days, the same as the Pregnancy Due Date Calculator.

A few practical notes

Confirm the date with your reproductive endocrinologist. They have your exact cycle day and the transfer record, and their date is the one that goes on the chart.

FET (frozen embryo transfer) is dated exactly the same way as a fresh transfer. What matters is the embryo’s age at transfer, not whether it was frozen in between, and not how long it stayed frozen.

Twins change the plan more than the date. ACOG’s 2021 guidance for twins with no other complications is delivery at 38 weeks when each baby has its own placenta, and somewhere from 34 to 37 weeks when they share one, so your obstetrician may work to an earlier target from fairly early on. And a due date is a midpoint, not an appointment: only about 4% of babies arrive on it, and a term delivery is anything from 37 to 42 weeks.