Conception Calculator Methodology

This page documents the formulas, input validation rules, and source citations used by the DueDateLab Conception 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 estimates a conception date from one of three reference points: last menstrual period (LMP), an existing estimated due date (EDD), or a known IVF event (embryo transfer or insemination). Conception is defined for this tool as the date of fertilization. The default method derives conception from LMP by assuming ovulation on cycle day 14 of a 28-day cycle, adjusted for user-entered cycle length. The result is always a single date plus a plausible fertile window, never a point prediction.

Formula 1, LMP method

Given an LMP date and cycle length:

Conception date = LMP + (cycle length − 14) days

For the standard 28-day cycle this reduces to LMP + 14 days. The 14-day offset reflects Lenton 1984's observed mean luteal-phase length of 14.13 ± 1.41 days across 327 ovulatory cycles, which is substantially more stable across individuals than the follicular phase. ACOG Committee Opinion 700 treats LMP-based dating as the default when no first-trimester ultrasound has been performed. NICE guideline NG201 takes a different default and recommends a dating ultrasound between 11 weeks 2 days and 14 weeks 1 day. The fertile-window output brackets this estimate with the six-day period ending on the day of ovulation, consistent with Wilcox et al. (NEJM 1995).

Formula 2, EDD method

Given an estimated due date:

Conception date = EDD − 266 days

This reverses Naegele's rule. It is the preferred method when a clinician has already issued a due date from a first-trimester dating ultrasound, because CRL-based ultrasound is more accurate than LMP in the first trimester per ACOG Committee Opinion 700. The ACOG-aligned ultrasound dating is inherited from the EDD without a second adjustment.

Formula 3, IVF-transfer method

For pregnancies achieved through IVF, fertilization happens in the laboratory before transfer. Given the transfer date and embryo age at transfer:

Conception date = Transfer date − Embryo age in days

Standard embryo ages are 3 days (cleavage-stage transfer) and 5 days (blastocyst transfer). Frozen embryo transfers use the same embryo age at freezing. This method is treated as ground truth when selected, because the fertilization event is directly known rather than estimated.

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.
• Estimated due date. Must be between 300 days in the future and 42 days in the past.
• Cycle length. Accepted range is 20 to 45 days. ACOG describes the typical adult menstrual cycle as 21 to 35 days in the Abnormal Uterine Bleeding FAQ; the calculator accepts a broader 20 to 45 range to accommodate irregular cycles and the broader 21 to 45 range that ACOG's Your First Period FAQ describes for the years after menarche.
• IVF transfer date. Must be on or before today and within the last 300 days.
• Embryo age. Accepted values are 3 (cleavage) or 5 (blastocyst). Values outside this range trigger a warning.

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:

• Most likely conception date as an ISO 8601 date.
• Fertile window (6-day interval ending on the estimated ovulation day).
• Gestational age today in completed weeks and days, derived from LMP where available.
• Implied estimated due date (conception date + 266 days) for user cross-reference.

Assumptions and limitations

The calculator assumes a singleton pregnancy and does not adjust for maternal age, BMI, parity, ethnicity, or obstetric history. For natural pregnancies it assumes ovulation on cycle day (cycle length − 14), which is a population median and can vary by several days between cycles. When cycle length is entered but ovulation timing was confirmed (for example by LH testing), we recommend using the Due Date Calculator with the known ovulation date instead. This is a dating tool, not a fertility-prediction or pregnancy-probability tool.

Privacy of calculations

All computation runs client-side in JavaScript. LMP date, EDD, cycle length, IVF transfer date, embryo age, 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 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.