Pregnancy Due Date Calculator
Estimate your estimated due date (EDD) and trimester milestones using Naegele’s rule — the 40-week (280-day) standard adopted by ACOG and used by midwives and obstetricians worldwide.
Estimated due date (40 weeks)
8 October 2026
- Estimated conception date
- 15 January 2026
- Start of 2nd trimester (14 weeks)
- 9 April 2026
- Start of 3rd trimester (28 weeks)
- 16 July 2026
Full-term pregnancy is 40 weeks (280 days) measured from the first day of the last menstrual period, per ACOG Committee Opinion 700 (Naegele's rule). About 4% of babies arrive on their due date; ~90% are born within two weeks either side, between 38 and 42 weeks. This is an educational estimate, not a medical assessment — talk to your midwife or doctor for a dating scan, which is more accurate than LMP after the first trimester.
How to use this calculator
Pick the method that fits how you know the date: first day of your last menstrual period (LMP) for natural conception, ovulation or intercourse date if you tracked it, or the transfer date if you had IVF (choose Day-3 embryo or Day-5 blastocyst). Enter the date and, if you are using the LMP method, your average cycle length. The calculator returns your estimated due date plus the dates that mark the start of your second and third trimesters.
How the calculation works
Naegele’s rule — the calculation American College of Obstetricians and Gynecologists (ACOG) Committee Opinion 700 still endorses — dates a 40-week pregnancy by adding 280 days to the first day of the last menstrual period, assuming a regular 28-day cycle and ovulation on day 14. Non-standard cycles shift the due date by (cycle length − 28) days; a 30-day cycle pushes EDD two days later, a 26-day cycle two days earlier. For IVF the embryo’s age is known precisely, so a Day-5 blastocyst transfer dates EDD as transfer date + 261 days, and a Day-3 embryo transfer as transfer + 263 days. For a known conception or ovulation date, EDD = conception + 266 days. All variants converge on the same fact: the average human gestation, measured from fertilisation, is roughly 266 days; from LMP it is 280 days.
Worked example
LMP = 1 January 2026, average cycle 28 days. EDD = 1 January 2026 + 280 days = 8 October 2026. Quick mental shortcut (Naegele’s short form): add 7 days, subtract 3 months, add 1 year. 1 Jan + 7 = 8 Jan; −3 months = 8 October the previous year; +1 year = 8 October 2026. Same answer. Trimester 2 begins at 14 weeks (9 April 2026), trimester 3 at 28 weeks (16 July 2026).
Frequently asked questions
How accurate is the due date my calculator gives me?
Naegele’s rule is a calendar estimate, not a measurement. Real gestation lengths vary: a 2013 study in Human Reproduction by Jukic et al. found that natural pregnancies measured from ovulation varied by up to 37 days between women. Only about 4% of babies arrive on their due date; roughly 90% are born within two weeks either side. For more accuracy, a first-trimester dating ultrasound (between 8 and 13 weeks) is typically within ± 5 days and supersedes the LMP estimate if the two disagree by more than a week.
Why does the IVF method give a different answer to LMP?
Because in IVF the date of fertilisation is known exactly — it is the day of egg retrieval, and the embryo’s age on transfer day is known (Day 3 or Day 5). LMP-based dating has to assume a regular 28-day cycle with ovulation on day 14, which is only roughly true for most women. ACOG recommends IVF dating take precedence over LMP whenever the IVF transfer date is known: EDD = transfer date + 266 − embryo age. For a Day-5 blastocyst, that is +261 days; for a Day-3 embryo, +263 days.
My cycle is not 28 days — does that matter?
Yes, for LMP-based dating it does. If you ovulate later than day 14 — which is what a longer cycle implies — conception happened later relative to LMP, so the due date is also later. The standard adjustment, used by RCOG and most digital tools, is to shift the EDD by (cycle length − 28) days. A 30-day cycle pushes EDD two days later; a 26-day cycle two days earlier. This adjustment is automatic when you enter your cycle length above. For very irregular cycles (variation greater than ± 5 days), a dating ultrasound is more reliable than the calendar.
What counts as the first day of my last period?
The first day you saw bleeding from your last period before becoming pregnant — not the day it ended, and not the day of the period before that. Spotting is generally not counted; use the day full menstrual bleeding started. If you are unsure of the exact day, use your best estimate and verify with a dating ultrasound at the first midwife or doctor appointment.
When do the trimesters start and end?
There is no single regulator-set boundary, but the most common convention — used by NHS, ACOG and the WHO — is: trimester 1 from 0 to 13 weeks 6 days, trimester 2 from 14 weeks 0 days to 27 weeks 6 days, and trimester 3 from 28 weeks 0 days to birth. Gestational weeks are counted from the first day of LMP, so a pregnancy is already 2 weeks "pregnant" at conception and 4 weeks at the first missed period. Some textbooks place the trimester boundaries at 13 and 27 weeks instead; the difference is a few days and does not affect clinical care.
Is a calculator due date the same as the date my baby will be born?
No. The due date is a statistical mid-point, not a deadline. In a large 2020 cohort study of 19 million US births, fewer than 5% delivered on the calculated due date, roughly 60% delivered within a week either side, and about 90% within two weeks. Doctors generally consider 37 to 42 weeks "term" — most spontaneous labours begin in this window. The due date is most useful as an anchor for scheduling antenatal scans, vaccinations and screening tests rather than as a prediction of the birthday itself.