Conception date plus 266 days
A pregnancy dated from the last period lasts 280 days, but the first two of those weeks happen before ovulation. Counting from conception removes them: due date = conception + 266 days. The calculator also shows the LMP-equivalent (14 days before conception) because that is the date on which clinics, scans, and week-by-week guides are based.
Ovulation, conception, and IUI dates are interchangeable here. The egg survives about a day after release, so fertilisation happens within that window whichever way the sperm arrived.
When to trust this over your last period
Last-period dating assumes ovulation on cycle day 14. With a 35-day cycle you probably ovulated around day 21, which pushes the real due date a week later than the period-based estimate. If you know your ovulation or conception date from an OPK, temperature shift, or a single act of intercourse, this calculator is closer to the truth. If you had an early scan, the due date by ultrasound calculator is more accurate still, and the IVF due date calculator handles embryo transfers.
Frequently asked questions
How is a due date calculated from the conception date?
Pregnancy lasts about 38 weeks from conception, so the due date is the conception date plus 266 days. The more familiar 40-week figure counts from the first day of the last period, which is roughly two weeks before conception. Both methods land on the same day for a typical cycle.
Is a due date from conception more accurate than from my last period?
It can be, if the conception date is genuinely known, for example from a single act of intercourse, an OPK peak, or an IUI. A last-period date assumes you ovulated on day 14, which is wrong for anyone with a long or short cycle. An early dating ultrasound is still the most accurate method of all.
Can I use my ovulation date instead?
Yes. The egg is fertilised within about 24 hours of ovulation, so ovulation date and conception date are the same for due date purposes. Pick the ovulation option in the calculator; the maths does not change.
What about IUI?
For intrauterine insemination, use the insemination date. Ovulation is triggered or confirmed around the procedure, so IUI date plus 266 days is the standard estimate. For IVF or a frozen embryo transfer, use the IVF due date calculator instead because the embryo is already several days old at transfer.
Why does my clinic give a different due date?
Clinics date pregnancies from the last menstrual period or from an early ultrasound, and they usually keep the ultrasound date if it differs from your dates by more than a few days. A few days either way is normal. Only about 5% of babies arrive on the exact due date.
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.