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.

The formulas

For a Day-3 embryo transfer: Due date = transfer date + 263 days. For a Day-5 blastocyst transfer, which is the more common one now: Due date = transfer date + 261 days.

Why they differ. A full embryo development period runs 266 days from fertilisation. 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, so 266 − 3 = 263. 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-3 transfer: equivalent LMP = 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 blastocyst, or 2 weeks and 3 days for a day-3 embryo. 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 transfer on day 3 instead would be due 3 December.

Milestones, counted from a Day-5 transfer date

Add 2 days to each for a Day-3 transfer. Trimester boundaries are a convention rather than a biological line, and you will see the first trimester quoted as ending anywhere from 12 to 14 weeks depending on the source. This page uses 13 and 28 weeks throughout, so the calculator and the table agree.

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.

IVF twins are commonly delivered around 36 to 38 weeks rather than 40, so your obstetrician may work to an earlier target from fairly early on. 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.