Due Date Calculator Methodology

This page documents the formulas, input validation rules, and source citations used by the DueDateLab Due Date Calculator. It exists for transparency: anyone can verify the formulas and sources behind each result. All calculations run in your browser, no inputs are sent to our servers.

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Summary

The calculator produces an estimated due date (EDD) from one of three reference points: last menstrual period (LMP), conception date, or a known due date. The default method is Naegele's rule, which adds 280 days to LMP. Conception-date input adds 266 days to conception. The calculator also supports an optional ultrasound override consistent with ACOG Committee Opinion 700. The result is always a single date plus a 37-42 week delivery window, never a point prediction.

Formula 1, LMP method (Naegele's rule)

Given an LMP date, the estimated due date is:

EDD = LMP + 280 days

This is mathematically equivalent to the classical statement of Naegele's rule, add one year, subtract three months, add seven days. The 280-day figure assumes a 28-day cycle with ovulation on cycle day 14 and gestation of 266 days from fertilization. It is the default estimation method documented in ACOG Committee Opinion 700 when a first-trimester ultrasound has not been performed. NICE guideline NG201 takes a different default: it recommends a dating ultrasound between 11 weeks 2 days and 14 weeks 1 day rather than an LMP-based formula.

Formula 2, conception-date method

Given a conception date, the estimated due date is:

EDD = Conception date + 266 days

This formula removes the 14-day ovulation offset that LMP-based dating assumes. It is used when the conception date is known with clinical certainty, for example after an IVF embryo transfer (EDD = transfer date + 266 days − embryo age in days), a timed insemination, or ovulation confirmed by LH testing. If the conception date is a guess rather than a measured event, clinical practice falls back to LMP-based dating.

Formula 3, reverse from a known due date

If a clinician has already issued a due date (for example after a dating ultrasound), the calculator works backward to return the implied LMP and conception date:

Implied LMP = EDD − 280 days
Implied conception = EDD − 266 days

This is useful when the patient wants to align home tracking apps or wearables to the clinician's dating without changing the EDD.

Ultrasound adjustment rules

A first-trimester ultrasound that measures crown-rump length (CRL) is more accurate than LMP-based dating. Following ACOG Committee Opinion 700:

• Up to 8 weeks 6 days, the ultrasound EDD replaces the LMP EDD if they disagree by more than 5 days.
• 9 weeks 0 days to 13 weeks 6 days, the ultrasound EDD replaces the LMP EDD if they disagree by more than 7 days.
• 14 weeks 0 days to 15 weeks 6 days, the ultrasound EDD replaces the LMP EDD if they disagree by more than 7 days.
• 16 weeks 0 days to 21 weeks 6 days, the ultrasound EDD replaces the LMP EDD if they disagree by more than 10 days.
• 22 weeks 0 days to 27 weeks 6 days, the ultrasound EDD replaces the LMP EDD if they disagree by more than 14 days.
• 28 weeks 0 days and later, ultrasound is the least accurate dating method (accuracy approximately ± 21 to 30 days per ACOG Committee Opinion 700). ACOG allows replacing the EDD only if the first ultrasound of the pregnancy is performed in the third trimester and disagrees with LMP dating by more than 21 days.

The DueDateLab calculator implements these thresholds as an optional override panel. If the user supplies a CRL-based ultrasound date and week of pregnancy at the time of scan, the calculator applies the ACOG threshold for that gestational window and either keeps the LMP EDD or replaces it.

Input validation

The calculator accepts only clinically plausible inputs and rejects values outside the following ranges:

• LMP date. Must be between 300 days ago and 14 days ago. LMP dates older than 300 days are post-term and outside routine clinical use. LMP dates in the last 14 days are implausible because a positive pregnancy test usually requires at least 10 to 14 days after conception.
• Conception date. Must be between 286 days ago and today. Conception dates older than 286 days are post-term. Future conception dates are rejected.
• Known due date. Must be between 300 days in the future and 42 days in the past (to handle recent deliveries where users still want to confirm the dating).
• Cycle length. Optional adjustment. Accepted range is 21 to 45 days. The calculator offsets LMP-based EDD by (cycle length − 28) days, a common clinical adjustment. Cycle length under 21 or over 45 is flagged as outside the normal population range.
• Ultrasound date. Must be on or before today and within the last 300 days.
• Gestational week at scan. Accepted range is 4 to 42 weeks.

Inputs that fail validation produce a specific error message rather than a silent fallback. The calculator never guesses at a missing input.

Outputs

Each calculation returns the following structured outputs:

• Estimated due date as an ISO 8601 date.
• Gestational age today in completed weeks and days.
• Days remaining to EDD.
• Trimester (1: 0 to 13 weeks 6 days, 2: 14 to 27 weeks 6 days, 3: 28 weeks and later).
• Full-term delivery window, 37 weeks 0 days to 42 weeks 0 days.
• Expected delivery window around the EDD. Healthy singleton gestational length varies by about 37 days between pregnancies, with a median of 268 days from ovulation (approximately 40 weeks 2 days from LMP) per Jukic et al. 2013. Parity, offspring sex, maternal BMI, and alcohol intake did not predict gestational length in that study.

Assumptions and limitations

The calculator assumes a singleton, naturally conceived pregnancy unless the user selects the IVF option. Twin, triplet, and higher-order pregnancies use the same EDD formulas but have different delivery expectations, and a clinician should advise on the expected timing. The calculator does not adjust for maternal age, BMI, parity, ethnicity, or obstetric history. It is a dating tool, not a risk-prediction tool. It is not a substitute for clinical evaluation, and it does not model miscarriage risk, pre-term labor risk, or any other outcome.

Privacy of calculations

All computation runs client-side in JavaScript. LMP date, conception date, cycle length, ultrasound date, and any other input never leaves the browser, is never stored in a cookie or local storage, and is never transmitted to a DueDateLab server or a third-party server. Analytics pages on the rest of the site are covered by the privacy policy, but the calculator page itself collects no input data.

Sources

Authorship

The DueDateLab Editorial team writes and maintains this page. Editorial is not a clinical practice. Every medical claim is bound to the primary citations listed above. The full source list at the end of the page documents the governing primary sources so readers can verify each statement against the original publication. DueDateLab is published from Belgium.

Conflicts of interest. DueDateLab is supported by advertising and may use affiliate links. Advertising and affiliate relationships do not influence the methodology, sources, or medical claims on this page. Any sponsored content is clearly labeled.

Last updated May 15, 2026.