IUI due date calculation: dating a pregnancy after insemination
Intrauterine insemination (IUI) sits neatly between natural conception and IVF when it comes to dating. Unlike natural conception, the timing is known — the insemination is scheduled around ovulation — so the due date can be worked out cleanly. Unlike IVF, there's no embryo transferred at a known developmental age, so the arithmetic is a touch simpler.
The short answer
Because IUI is timed to ovulation, the insemination date stands in for the conception date. So:
due date = IUI date + 266 days
That's the same 266-day figure used for conception dating, and it corresponds to an equivalent last menstrual period about 14 days before the insemination. Our conception mode handles this directly — enter the IUI date as the conception date and read off the due date and gestational age.
Why 266 days?
A full-term pregnancy runs about 280 days from the last menstrual period, and ovulation falls roughly 14 days after that period starts on a typical cycle. Since IUI is deliberately timed to ovulation, counting from the insemination skips those first two weeks — leaving 280 − 14 = 266 days. This is the same logic explained in conception date and due date.
IUI vs. IVF dating
The two are close but not identical. IVF transfers an embryo of a known age (day-3 or day-5), so its dating subtracts that embryo age; see IVF due date calculation. IUI places sperm before fertilisation, so fertilisation happens in the body within about a day of the procedure — which is why IUI is dated like a conception date rather than like a transfer. In practice, an IUI due date and a natural conception due date use exactly the same formula.
How reliable is an IUI due date?
More reliable than guessing from a last period alone, because the ovulation timing is known rather than assumed. It's slightly less precise than IVF, where fertilisation timing is exact, but the difference is small — usually a day or two. As with every method, an early ultrasound remains the reference standard and may gently adjust the date; see how accurate is your due date. Your fertility clinic will confirm dating at your first scan.
A worked example
If the insemination was on 1 March 2026, the estimated due date is 1 March + 266 days, which the calculator will place in early December 2026, along with the current gestational age and each milestone date. Enter your own IUI date to see the full timeline.
Frequently asked questions
How do I calculate my due date after IUI?
Add 266 days to the insemination date. Because IUI is timed to ovulation, the procedure date acts as the conception date, which is 266 days before the due date.
Is an IUI due date the same as an IVF due date?
The formula differs slightly. IUI is dated like a conception date (+266 days), while IVF subtracts the embryo's age at transfer. For IUI, use conception dating.
How accurate is an IUI due date?
Quite accurate, because the ovulation timing is known rather than estimated. An early ultrasound is still the reference standard and may adjust the date by a day or two.