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Pregnancy Due Date Calculator
Estimate your baby's due date from LMP or conception date. Trimesters, milestones, key appointment weeks. No signup — your inputs stay in your browser.
Step By Step
How to Use This Calculator
4 steps
- Choose 'Last Period (LMP)' if you know the first day of your last period — this is the most common method used in clinical practice.
- If you are certain of your conception date (e.g., from IVF or confirmed ovulation tracking), switch to 'Conception Date' mode.
- Enter your average cycle length. If you are unsure, 28 days is the medical default.
- Review the milestone timeline for key prenatal appointment weeks and trimester transitions.
Worked Example
Example: LMP 1 September 2025, 28-day cycle
Use this sample to sanity-check your inputs and understand what the final result represents.
- 1LMP: 1 September 2025
- 2Add 280 days: EDD = 8 June 2026
- 3Week 12 (end of first trimester): approximately 24 November 2025
- 4Week 20 anatomy scan: approximately 19 January 2026
Final Result
Estimated Due Date: 8 June 2026. Gestational age tracked from LMP onwards.
Methodology
Naegele's Rule (1830) — the global clinical standard
This section explains the calculation logic, assumptions, and source material used to make the result more trustworthy and easier to verify.
EDD = LMP + 280 days (for a 28-day cycle). Cycle adjustment: add (cycle length − 28) days to account for later or earlier ovulation. Conception date method: EDD = Conception date + 266 days (38 weeks), because 266 days from conception equals approximately 280 days from LMP. Reference: Jukic et al. (2013), Human Reproduction, confirmed average gestation of 280 days ± 2 weeks from LMP in spontaneous pregnancies.
Frequently Asked Questions
How is a pregnancy due date calculated?+
The standard method is Naegele's rule, described by Franz Karl Naegele in 1830: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. For cycles shorter or longer than 28 days, the formula is adjusted by the difference — if your cycle is 30 days, two extra days are added. A large 2013 study published in Human Reproduction (Jukic et al.) analysed spontaneous pregnancies and confirmed that the average gestation from LMP is about 280 days, though individual variation of ±2 weeks is normal.
How accurate is an LMP-based due date?+
An LMP-based estimate is accurate to within roughly ±2 weeks for most women. Studies show only about 5% of babies are born exactly on their estimated due date. A first-trimester crown-rump length (CRL) ultrasound scan between 8 and 13 weeks 6 days is substantially more accurate — the ACOG and SOGC guidelines recommend using the ultrasound date when it differs from the LMP date by more than 5–7 days in the first trimester.
What is gestational age and how is it counted?+
Gestational age is measured in weeks and days from the first day of the LMP, not from the conception date. This convention means the clock starts approximately 2 weeks before fertilisation. So a pregnancy at '6 weeks gestational age' is actually about 4 weeks from conception. Clinicians, ultrasound reports, and pregnancy apps all use gestational age by this convention — the same standard followed by the WHO and ACOG.
What are the three trimesters?+
The first trimester runs from week 1 to the end of week 12 (roughly 0–13 weeks). Major organ formation occurs during this period. The second trimester spans weeks 13 through 27. The third trimester begins at week 28 and ends at delivery. These divisions are clinical conventions used worldwide; the WHO defines early term as 37–38 weeks, full term as 39–40 weeks, and late term as 41 weeks.
Why does the calculator ask for cycle length?+
Naegele's rule assumes ovulation on day 14 of a 28-day cycle. If your cycle is consistently longer — say, 35 days — you probably ovulate around day 21, meaning conception happened later than a 28-day calculator would assume. The calculator adjusts the EDD by adding the extra days (35 − 28 = 7 days in this example), which brings the estimate closer to your actual ovulation timing.
Is this calculator a substitute for an ultrasound?+
No. This is a planning tool, not a clinical instrument. Only an ultrasound performed by a trained sonographer can confirm gestational age and exclude ectopic pregnancy or other complications. Always consult a qualified healthcare provider — an OB-GYN, midwife, or GP — early in your pregnancy to establish care and get a clinical due date.