How to use this calculator
- Enter or choose calculate from.
- The estimated due date updates as you type. Use Copy result or Share to keep it.
Method
LMP: due = LMP + (cycle − 28) + 280; Conception: due = conception + 266; IVF day 5: transfer + 261; IVF day 3: transfer + 263; Scan: due = scan date + 280 − gestational age at scan (days)
- LMP
- first day of last period
- 280 days
- 40 weeks
- gestational age at scan
- weeks × 7 + days, as measured by ultrasound
Worked example
- Last period started 1 March 2026 and the cycle is 28 days, so no adjustment is needed.
- Due date = 1 March 2026 + 280 days = 6 December 2026.
- Term runs from 37 weeks (+259 days, 15 November) to 41 weeks 6 days (+293 days, 19 December).
A last period starting on 1 March 2026 gives an estimated due date of Sunday 6 December 2026.
What the result means
IVF dates are the most precise because the embryo’s age is known. Otherwise, a first-trimester scan is more accurate than period dates, and your care team will normally use it to confirm or change the due date.
Assumptions and limits
- Conventions from ACOG Committee Opinion 700: 280 days from LMP; 261 days from a day-5 and 263 days from a day-3 embryo transfer.
- Conception taken as 14 days after the (cycle-adjusted) LMP, 266 days before the due date.
- Period-based dates assume regular cycles with ovulation 14 days before the next period.
- The scan method assumes the gestational age you enter is the one reported on your scan.
Questions
What is Naegele’s rule?
It is a shortcut for the same calculation: take the first day of the last period, subtract three months and add seven days (and a year where needed).
Why does a day-5 transfer use 261 days?
A day-5 embryo is already 5 days past fertilisation, and fertilisation corresponds to 2 weeks of gestational age, so 280 − 14 − 5 = 261 days remain.
Sources
Please note: This is a general screening estimate for adults, not medical advice or a diagnosis. Talk to a qualified health professional about your health.